Add the Drop exemplar Reservoir implementation (#4850)

sdk/metric/internal/exemplar/drop.go

@@ -0,0 +1,41 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package exemplar // import "go.opentelemetry.io/otel/sdk/metric/internal/exemplar"
+
+import (
+	"context"
+	"time"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+)
+
+// Drop returns a [Reservoir] that drops all measurements it is offered.
+func Drop[N int64 | float64]() Reservoir[N] { return &dropRes[N]{} }
+
+type dropRes[N int64 | float64] struct{}
+
+// Offer does nothing, all measurements offered will be dropped.
+func (r *dropRes[N]) Offer(context.Context, time.Time, N, []attribute.KeyValue) {}
+
+// Collect resets dest. No exemplars will ever be returned.
+func (r *dropRes[N]) Collect(dest *[]metricdata.Exemplar[N]) {
+	*dest = (*dest)[:0]
+}
+
+// Flush resets dest. No exemplars will ever be returned.
+func (r *dropRes[N]) Flush(dest *[]metricdata.Exemplar[N]) {
+	*dest = (*dest)[:0]
+}

sdk/metric/internal/exemplar/drop_test.go

@@ -0,0 +1,29 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package exemplar
+
+import (
+	"testing"
+)
+
+func TestDrop(t *testing.T) {
+	t.Run("Int64", ReservoirTest[int64](func(int) (Reservoir[int64], int) {
+		return Drop[int64](), 0
+	}))
+
+	t.Run("Float64", ReservoirTest[float64](func(int) (Reservoir[float64], int) {
+		return Drop[float64](), 0
+	}))
+}