diff --git a/README.md b/README.md
index e100975..61552be 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,6 @@
+_(If you are reading this because you are interested in Prometheus's native
+histograms, pay special attention to the last paragraph below.)_
+
 # Deprecation note
 
 This repository used to contain the [protocol
@@ -32,3 +35,10 @@ for the protobuf-based exposition format.
 
 Check out the [OpenMetrics project](https://openmetrics.io/) for the future of
 the data model and exposition format used by Prometheus and others.
+
+Note, though, that in an ironic twist of fate, the protobuf-based exposition
+format got revived to ease the implementation of experimental support for
+native histograms in Prometheus. Therefore, starting with v2.40.0, the
+Prometheus server is again capable of ingesting the protobuf-based exposition
+format (if the respective feature flag is enabled). Eventually, native
+histogram support will be added in some form to OpenMetrics, too.
diff --git a/go/metrics.pb.go b/go/metrics.pb.go
index a255337..35904ea 100644
--- a/go/metrics.pb.go
+++ b/go/metrics.pb.go
@@ -24,11 +24,18 @@ const _ = proto.ProtoPackageIsVersion3 // please upgrade the proto package
 type MetricType int32
 
 const (
-	MetricType_COUNTER   MetricType = 0
-	MetricType_GAUGE     MetricType = 1
-	MetricType_SUMMARY   MetricType = 2
-	MetricType_UNTYPED   MetricType = 3
+	// COUNTER must use the Metric field "counter".
+	MetricType_COUNTER MetricType = 0
+	// GAUGE must use the Metric field "gauge".
+	MetricType_GAUGE MetricType = 1
+	// SUMMARY must use the Metric field "summary".
+	MetricType_SUMMARY MetricType = 2
+	// UNTYPED must use the Metric field "untyped".
+	MetricType_UNTYPED MetricType = 3
+	// HISTOGRAM must use the Metric field "histogram".
 	MetricType_HISTOGRAM MetricType = 4
+	// GAUGE_HISTOGRAM must use the Metric field "histogram".
+	MetricType_GAUGE_HISTOGRAM MetricType = 5
 )
 
 var MetricType_name = map[int32]string{
@@ -37,14 +44,16 @@ var MetricType_name = map[int32]string{
 	2: "SUMMARY",
 	3: "UNTYPED",
 	4: "HISTOGRAM",
+	5: "GAUGE_HISTOGRAM",
 }
 
 var MetricType_value = map[string]int32{
-	"COUNTER":   0,
-	"GAUGE":     1,
-	"SUMMARY":   2,
-	"UNTYPED":   3,
-	"HISTOGRAM": 4,
+	"COUNTER":         0,
+	"GAUGE":           1,
+	"SUMMARY":         2,
+	"UNTYPED":         3,
+	"HISTOGRAM":       4,
+	"GAUGE_HISTOGRAM": 5,
 }
 
 func (x MetricType) Enum() *MetricType {
@@ -345,9 +354,34 @@ func (m *Untyped) GetValue() float64 {
 }
 
 type Histogram struct {
-	SampleCount          *uint64   `protobuf:"varint,1,opt,name=sample_count,json=sampleCount" json:"sample_count,omitempty"`
-	SampleSum            *float64  `protobuf:"fixed64,2,opt,name=sample_sum,json=sampleSum" json:"sample_sum,omitempty"`
-	Bucket               []*Bucket `protobuf:"bytes,3,rep,name=bucket" json:"bucket,omitempty"`
+	SampleCount      *uint64  `protobuf:"varint,1,opt,name=sample_count,json=sampleCount" json:"sample_count,omitempty"`
+	SampleCountFloat *float64 `protobuf:"fixed64,4,opt,name=sample_count_float,json=sampleCountFloat" json:"sample_count_float,omitempty"`
+	SampleSum        *float64 `protobuf:"fixed64,2,opt,name=sample_sum,json=sampleSum" json:"sample_sum,omitempty"`
+	// Buckets for the conventional histogram.
+	Bucket []*Bucket `protobuf:"bytes,3,rep,name=bucket" json:"bucket,omitempty"`
+	// schema defines the bucket schema. Currently, valid numbers are -4 <= n <= 8.
+	// They are all for base-2 bucket schemas, where 1 is a bucket boundary in each case, and
+	// then each power of two is divided into 2^n logarithmic buckets.
+	// Or in other words, each bucket boundary is the previous boundary times 2^(2^-n).
+	// In the future, more bucket schemas may be added using numbers < -4 or > 8.
+	Schema         *int32   `protobuf:"zigzag32,5,opt,name=schema" json:"schema,omitempty"`
+	ZeroThreshold  *float64 `protobuf:"fixed64,6,opt,name=zero_threshold,json=zeroThreshold" json:"zero_threshold,omitempty"`
+	ZeroCount      *uint64  `protobuf:"varint,7,opt,name=zero_count,json=zeroCount" json:"zero_count,omitempty"`
+	ZeroCountFloat *float64 `protobuf:"fixed64,8,opt,name=zero_count_float,json=zeroCountFloat" json:"zero_count_float,omitempty"`
+	// Negative buckets for the native histogram.
+	NegativeSpan []*BucketSpan `protobuf:"bytes,9,rep,name=negative_span,json=negativeSpan" json:"negative_span,omitempty"`
+	// Use either "negative_delta" or "negative_count", the former for
+	// regular histograms with integer counts, the latter for float
+	// histograms.
+	NegativeDelta []int64   `protobuf:"zigzag64,10,rep,name=negative_delta,json=negativeDelta" json:"negative_delta,omitempty"`
+	NegativeCount []float64 `protobuf:"fixed64,11,rep,name=negative_count,json=negativeCount" json:"negative_count,omitempty"`
+	// Positive buckets for the native histogram.
+	PositiveSpan []*BucketSpan `protobuf:"bytes,12,rep,name=positive_span,json=positiveSpan" json:"positive_span,omitempty"`
+	// Use either "positive_delta" or "positive_count", the former for
+	// regular histograms with integer counts, the latter for float
+	// histograms.
+	PositiveDelta        []int64   `protobuf:"zigzag64,13,rep,name=positive_delta,json=positiveDelta" json:"positive_delta,omitempty"`
+	PositiveCount        []float64 `protobuf:"fixed64,14,rep,name=positive_count,json=positiveCount" json:"positive_count,omitempty"`
 	XXX_NoUnkeyedLiteral struct{}  `json:"-"`
 	XXX_unrecognized     []byte    `json:"-"`
 	XXX_sizecache        int32     `json:"-"`
@@ -385,6 +419,13 @@ func (m *Histogram) GetSampleCount() uint64 {
 	return 0
 }
 
+func (m *Histogram) GetSampleCountFloat() float64 {
+	if m != nil && m.SampleCountFloat != nil {
+		return *m.SampleCountFloat
+	}
+	return 0
+}
+
 func (m *Histogram) GetSampleSum() float64 {
 	if m != nil && m.SampleSum != nil {
 		return *m.SampleSum
@@ -399,8 +440,81 @@ func (m *Histogram) GetBucket() []*Bucket {
 	return nil
 }
 
+func (m *Histogram) GetSchema() int32 {
+	if m != nil && m.Schema != nil {
+		return *m.Schema
+	}
+	return 0
+}
+
+func (m *Histogram) GetZeroThreshold() float64 {
+	if m != nil && m.ZeroThreshold != nil {
+		return *m.ZeroThreshold
+	}
+	return 0
+}
+
+func (m *Histogram) GetZeroCount() uint64 {
+	if m != nil && m.ZeroCount != nil {
+		return *m.ZeroCount
+	}
+	return 0
+}
+
+func (m *Histogram) GetZeroCountFloat() float64 {
+	if m != nil && m.ZeroCountFloat != nil {
+		return *m.ZeroCountFloat
+	}
+	return 0
+}
+
+func (m *Histogram) GetNegativeSpan() []*BucketSpan {
+	if m != nil {
+		return m.NegativeSpan
+	}
+	return nil
+}
+
+func (m *Histogram) GetNegativeDelta() []int64 {
+	if m != nil {
+		return m.NegativeDelta
+	}
+	return nil
+}
+
+func (m *Histogram) GetNegativeCount() []float64 {
+	if m != nil {
+		return m.NegativeCount
+	}
+	return nil
+}
+
+func (m *Histogram) GetPositiveSpan() []*BucketSpan {
+	if m != nil {
+		return m.PositiveSpan
+	}
+	return nil
+}
+
+func (m *Histogram) GetPositiveDelta() []int64 {
+	if m != nil {
+		return m.PositiveDelta
+	}
+	return nil
+}
+
+func (m *Histogram) GetPositiveCount() []float64 {
+	if m != nil {
+		return m.PositiveCount
+	}
+	return nil
+}
+
+// A Bucket of a conventional histogram, each of which is treated as
+// an individual counter-like time series by Prometheus.
 type Bucket struct {
 	CumulativeCount      *uint64   `protobuf:"varint,1,opt,name=cumulative_count,json=cumulativeCount" json:"cumulative_count,omitempty"`
+	CumulativeCountFloat *float64  `protobuf:"fixed64,4,opt,name=cumulative_count_float,json=cumulativeCountFloat" json:"cumulative_count_float,omitempty"`
 	UpperBound           *float64  `protobuf:"fixed64,2,opt,name=upper_bound,json=upperBound" json:"upper_bound,omitempty"`
 	Exemplar             *Exemplar `protobuf:"bytes,3,opt,name=exemplar" json:"exemplar,omitempty"`
 	XXX_NoUnkeyedLiteral struct{}  `json:"-"`
@@ -440,6 +554,13 @@ func (m *Bucket) GetCumulativeCount() uint64 {
 	return 0
 }
 
+func (m *Bucket) GetCumulativeCountFloat() float64 {
+	if m != nil && m.CumulativeCountFloat != nil {
+		return *m.CumulativeCountFloat
+	}
+	return 0
+}
+
 func (m *Bucket) GetUpperBound() float64 {
 	if m != nil && m.UpperBound != nil {
 		return *m.UpperBound
@@ -454,6 +575,59 @@ func (m *Bucket) GetExemplar() *Exemplar {
 	return nil
 }
 
+// A BucketSpan defines a number of consecutive buckets in a native
+// histogram with their offset. Logically, it would be more
+// straightforward to include the bucket counts in the Span. However,
+// the protobuf representation is more compact in the way the data is
+// structured here (with all the buckets in a single array separate
+// from the Spans).
+type BucketSpan struct {
+	Offset               *int32   `protobuf:"zigzag32,1,opt,name=offset" json:"offset,omitempty"`
+	Length               *uint32  `protobuf:"varint,2,opt,name=length" json:"length,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *BucketSpan) Reset()         { *m = BucketSpan{} }
+func (m *BucketSpan) String() string { return proto.CompactTextString(m) }
+func (*BucketSpan) ProtoMessage()    {}
+func (*BucketSpan) Descriptor() ([]byte, []int) {
+	return fileDescriptor_d1e5ddb18987a258, []int{8}
+}
+
+func (m *BucketSpan) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_BucketSpan.Unmarshal(m, b)
+}
+func (m *BucketSpan) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_BucketSpan.Marshal(b, m, deterministic)
+}
+func (m *BucketSpan) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_BucketSpan.Merge(m, src)
+}
+func (m *BucketSpan) XXX_Size() int {
+	return xxx_messageInfo_BucketSpan.Size(m)
+}
+func (m *BucketSpan) XXX_DiscardUnknown() {
+	xxx_messageInfo_BucketSpan.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_BucketSpan proto.InternalMessageInfo
+
+func (m *BucketSpan) GetOffset() int32 {
+	if m != nil && m.Offset != nil {
+		return *m.Offset
+	}
+	return 0
+}
+
+func (m *BucketSpan) GetLength() uint32 {
+	if m != nil && m.Length != nil {
+		return *m.Length
+	}
+	return 0
+}
+
 type Exemplar struct {
 	Label                []*LabelPair         `protobuf:"bytes,1,rep,name=label" json:"label,omitempty"`
 	Value                *float64             `protobuf:"fixed64,2,opt,name=value" json:"value,omitempty"`
@@ -467,7 +641,7 @@ func (m *Exemplar) Reset()         { *m = Exemplar{} }
 func (m *Exemplar) String() string { return proto.CompactTextString(m) }
 func (*Exemplar) ProtoMessage()    {}
 func (*Exemplar) Descriptor() ([]byte, []int) {
-	return fileDescriptor_d1e5ddb18987a258, []int{8}
+	return fileDescriptor_d1e5ddb18987a258, []int{9}
 }
 
 func (m *Exemplar) XXX_Unmarshal(b []byte) error {
@@ -526,7 +700,7 @@ func (m *Metric) Reset()         { *m = Metric{} }
 func (m *Metric) String() string { return proto.CompactTextString(m) }
 func (*Metric) ProtoMessage()    {}
 func (*Metric) Descriptor() ([]byte, []int) {
-	return fileDescriptor_d1e5ddb18987a258, []int{9}
+	return fileDescriptor_d1e5ddb18987a258, []int{10}
 }
 
 func (m *Metric) XXX_Unmarshal(b []byte) error {
@@ -610,7 +784,7 @@ func (m *MetricFamily) Reset()         { *m = MetricFamily{} }
 func (m *MetricFamily) String() string { return proto.CompactTextString(m) }
 func (*MetricFamily) ProtoMessage()    {}
 func (*MetricFamily) Descriptor() ([]byte, []int) {
-	return fileDescriptor_d1e5ddb18987a258, []int{10}
+	return fileDescriptor_d1e5ddb18987a258, []int{11}
 }
 
 func (m *MetricFamily) XXX_Unmarshal(b []byte) error {
@@ -669,6 +843,7 @@ func init() {
 	proto.RegisterType((*Untyped)(nil), "io.prometheus.client.Untyped")
 	proto.RegisterType((*Histogram)(nil), "io.prometheus.client.Histogram")
 	proto.RegisterType((*Bucket)(nil), "io.prometheus.client.Bucket")
+	proto.RegisterType((*BucketSpan)(nil), "io.prometheus.client.BucketSpan")
 	proto.RegisterType((*Exemplar)(nil), "io.prometheus.client.Exemplar")
 	proto.RegisterType((*Metric)(nil), "io.prometheus.client.Metric")
 	proto.RegisterType((*MetricFamily)(nil), "io.prometheus.client.MetricFamily")
@@ -679,47 +854,61 @@ func init() {
 }
 
 var fileDescriptor_d1e5ddb18987a258 = []byte{
-	// 672 bytes of a gzipped FileDescriptorProto
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+	// 896 bytes of a gzipped FileDescriptorProto
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+	0xfc, 0xbf, 0xac, 0x9d, 0x41, 0x77, 0x55, 0xae, 0x7f, 0xa2, 0xc2, 0x18, 0x7b, 0xbf, 0xd9, 0xd8,
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+	0xef, 0x62, 0x53, 0xa1, 0x8f, 0xe0, 0xe0, 0x57, 0x9a, 0xf3, 0x50, 0x6c, 0x73, 0x5a, 0x6c, 0x79,
+	0xb2, 0xf1, 0xba, 0x6a, 0xc3, 0xa1, 0x44, 0x17, 0x16, 0x94, 0x9a, 0x14, 0x4d, 0x5b, 0xec, 0x29,
+	0x8b, 0xae, 0x44, 0xb4, 0xc1, 0x53, 0x18, 0xd5, 0xcb, 0xc6, 0x5e, 0x5f, 0xcd, 0x39, 0xa8, 0x48,
+	0xda, 0xdc, 0x14, 0x86, 0x29, 0x8d, 0x88, 0x88, 0x5f, 0xd2, 0xb0, 0xc8, 0x48, 0xea, 0xb9, 0xca,
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+	0xd0, 0x44, 0x10, 0x0f, 0xc6, 0xad, 0x53, 0x84, 0xab, 0xe1, 0x8f, 0x24, 0x78, 0x8b, 0xa6, 0xa5,
+	0x0f, 0xc6, 0x2d, 0xe9, 0xce, 0xa2, 0x5a, 0xfe, 0x14, 0x86, 0x19, 0x2f, 0xe2, 0x5a, 0xd4, 0xfe,
+	0x9b, 0x8a, 0xb2, 0x6d, 0x56, 0x54, 0x35, 0x46, 0x8b, 0x1a, 0x6a, 0x51, 0x16, 0xad, 0x44, 0x55,
+	0x34, 0x2d, 0xea, 0x40, 0x8b, 0xb2, 0xa8, 0x12, 0xe5, 0xff, 0xe9, 0x40, 0x57, 0x6f, 0x85, 0x3e,
+	0x86, 0xd1, 0xba, 0x64, 0x65, 0x72, 0xd3, 0x88, 0xbe, 0x66, 0x77, 0x6a, 0x5c, 0x5b, 0x39, 0x83,
+	0x7b, 0xaf, 0x52, 0x6f, 0x5d, 0xb7, 0xc3, 0x57, 0x1a, 0xf4, 0x5b, 0x39, 0x81, 0x41, 0x99, 0x65,
+	0x34, 0x0f, 0x57, 0xbc, 0x4c, 0x37, 0xe6, 0xce, 0x81, 0x82, 0x2e, 0x24, 0x72, 0x2b, 0x17, 0x5a,
+	0xff, 0x3b, 0x17, 0xa0, 0x3e, 0x32, 0x79, 0x11, 0xf9, 0xd5, 0x55, 0x41, 0xb5, 0x83, 0xbb, 0xd8,
+	0x54, 0x12, 0x4f, 0x68, 0x1a, 0x89, 0xad, 0xda, 0x7d, 0x88, 0x4d, 0xe5, 0xff, 0xee, 0x40, 0xdf,
+	0x0e, 0x45, 0xf7, 0xa1, 0x93, 0xc8, 0x54, 0xf4, 0x1c, 0xf5, 0x82, 0x4e, 0x9a, 0x35, 0x54, 0xc1,
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+	0x03, 0x9b, 0xcb, 0xc1, 0xc2, 0x32, 0x70, 0x4d, 0xf6, 0xff, 0xde, 0x83, 0xee, 0x4c, 0xa5, 0xfc,
+	0xdb, 0x2a, 0xfa, 0x0c, 0x3a, 0x91, 0xcc, 0x69, 0x13, 0xb2, 0xef, 0x35, 0xb7, 0xa9, 0x28, 0xc7,
+	0x9a, 0x89, 0xbe, 0x80, 0xde, 0x5a, 0x67, 0xb7, 0x11, 0x7b, 0xdc, 0xdc, 0x64, 0x02, 0x1e, 0x5b,
+	0xb6, 0x6c, 0x2c, 0x74, 0xb0, 0xaa, 0x3b, 0xb0, 0xb3, 0xd1, 0xa4, 0x2f, 0xb6, 0x6c, 0xd9, 0x58,
+	0xea, 0x20, 0x54, 0xa1, 0xb1, 0xb3, 0xd1, 0xa4, 0x25, 0xb6, 0x6c, 0xf4, 0x0d, 0xb8, 0x5b, 0x9b,
+	0x8f, 0x2a, 0x2c, 0x76, 0x1e, 0x4c, 0x15, 0xa3, 0xb8, 0xee, 0x90, 0x89, 0x5a, 0x9d, 0x75, 0xc8,
+	0x0a, 0x95, 0x48, 0x2d, 0x3c, 0xa8, 0xb0, 0x59, 0xe1, 0xff, 0xe1, 0xc0, 0xbe, 0x7e, 0x03, 0x8f,
+	0x09, 0x8b, 0x93, 0xeb, 0xc6, 0x4f, 0x24, 0x82, 0xf6, 0x96, 0x26, 0x99, 0xf9, 0x42, 0xaa, 0x67,
+	0x74, 0x06, 0x6d, 0xa9, 0x51, 0x1d, 0xe1, 0xc1, 0xae, 0x5f, 0xb8, 0x9e, 0xbc, 0xb8, 0xce, 0x28,
+	0x56, 0x6c, 0x99, 0xb9, 0xfa, 0xab, 0xee, 0xb5, 0x5f, 0x97, 0xb9, 0xba, 0x0f, 0x1b, 0xee, 0x27,
+	0x2b, 0x80, 0x7a, 0x12, 0x1a, 0x40, 0xef, 0xe1, 0xb3, 0xe5, 0xd3, 0xc5, 0x14, 0x8f, 0xde, 0x41,
+	0x2e, 0x74, 0x2e, 0xcf, 0x97, 0x97, 0xd3, 0x91, 0x23, 0xf1, 0xf9, 0x72, 0x36, 0x3b, 0xc7, 0x2f,
+	0x46, 0x7b, 0xb2, 0x58, 0x3e, 0x5d, 0xbc, 0x78, 0x3e, 0x7d, 0x34, 0x6a, 0xa1, 0x21, 0xb8, 0x4f,
+	0xbe, 0x9d, 0x2f, 0x9e, 0x5d, 0xe2, 0xf3, 0xd9, 0xa8, 0x8d, 0xde, 0x85, 0x3b, 0xaa, 0x27, 0xac,
+	0xc1, 0xce, 0x05, 0x86, 0xc6, 0x3f, 0x18, 0x3f, 0x3c, 0x88, 0x62, 0xb1, 0x2d, 0x57, 0xc1, 0x9a,
+	0xb3, 0x7f, 0xff, 0x45, 0x09, 0x19, 0xdf, 0xd0, 0x64, 0x12, 0xf1, 0xaf, 0x62, 0x1e, 0xd6, 0xab,
+	0xa1, 0x5e, 0xfd, 0x27, 0x00, 0x00, 0xff, 0xff, 0x16, 0x77, 0x81, 0x98, 0xd7, 0x08, 0x00, 0x00,
 }
diff --git a/io/prometheus/client/metrics.proto b/io/prometheus/client/metrics.proto
index c9546f1..778ebdc 100644
--- a/io/prometheus/client/metrics.proto
+++ b/io/prometheus/client/metrics.proto
@@ -25,11 +25,18 @@ message LabelPair {
 }
 
 enum MetricType {
-  COUNTER    = 0;
-  GAUGE      = 1;
-  SUMMARY    = 2;
-  UNTYPED    = 3;
-  HISTOGRAM  = 4;
+  // COUNTER must use the Metric field "counter".
+  COUNTER = 0;
+  // GAUGE must use the Metric field "gauge".
+  GAUGE = 1;
+  // SUMMARY must use the Metric field "summary".
+  SUMMARY = 2;
+  // UNTYPED must use the Metric field "untyped".
+  UNTYPED = 3;
+  // HISTOGRAM must use the Metric field "histogram".
+  HISTOGRAM = 4;
+  // GAUGE_HISTOGRAM must use the Metric field "histogram".
+  GAUGE_HISTOGRAM = 5;
 }
 
 message Gauge {
@@ -57,15 +64,60 @@ message Untyped {
 }
 
 message Histogram {
-  optional uint64 sample_count = 1;
-  optional double sample_sum   = 2;
-  repeated Bucket bucket       = 3; // Ordered in increasing order of upper_bound, +Inf bucket is optional.
+  optional uint64 sample_count       = 1;
+  optional double sample_count_float = 4; // Overrides sample_count if > 0.
+  optional double sample_sum         = 2;
+  // Buckets for the conventional histogram.
+  repeated Bucket bucket             = 3; // Ordered in increasing order of upper_bound, +Inf bucket is optional.
+
+  // Everything below here is for native histograms (also known as sparse histograms).
+  // Native histograms are an experimental feature without stability guarantees.
+
+  // schema defines the bucket schema. Currently, valid numbers are -4 <= n <= 8.
+  // They are all for base-2 bucket schemas, where 1 is a bucket boundary in each case, and
+  // then each power of two is divided into 2^n logarithmic buckets.
+  // Or in other words, each bucket boundary is the previous boundary times 2^(2^-n).
+  // In the future, more bucket schemas may be added using numbers < -4 or > 8.
+  optional sint32 schema           = 5;
+  optional double zero_threshold   = 6; // Breadth of the zero bucket.
+  optional uint64 zero_count       = 7; // Count in zero bucket.
+  optional double zero_count_float = 8; // Overrides sb_zero_count if > 0.
+
+  // Negative buckets for the native histogram.
+  repeated BucketSpan negative_span =  9;
+  // Use either "negative_delta" or "negative_count", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 negative_delta    = 10; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double negative_count    = 11; // Absolute count of each bucket.
+
+  // Positive buckets for the native histogram.
+  repeated BucketSpan positive_span = 12;
+  // Use either "positive_delta" or "positive_count", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 positive_delta    = 13; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double positive_count    = 14; // Absolute count of each bucket.
 }
 
+// A Bucket of a conventional histogram, each of which is treated as
+// an individual counter-like time series by Prometheus.
 message Bucket {
-  optional uint64   cumulative_count = 1; // Cumulative in increasing order.
-  optional double   upper_bound      = 2; // Inclusive.
-  optional Exemplar exemplar         = 3;
+  optional uint64   cumulative_count       = 1; // Cumulative in increasing order.
+  optional double   cumulative_count_float = 4; // Overrides cumulative_count if > 0.
+  optional double   upper_bound            = 2; // Inclusive.
+  optional Exemplar exemplar               = 3;
+}
+
+// A BucketSpan defines a number of consecutive buckets in a native
+// histogram with their offset. Logically, it would be more
+// straightforward to include the bucket counts in the Span. However,
+// the protobuf representation is more compact in the way the data is
+// structured here (with all the buckets in a single array separate
+// from the Spans).
+message BucketSpan {
+  optional sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
+  optional uint32 length = 2; // Length of consecutive buckets.
 }
 
 message Exemplar {