Include explicitly sampled in spans

Author: DomParfitt

o11y/honeycomb/sampler.go

@@ -18,6 +18,13 @@ type TraceSampler struct {
 
 // Hook implements beeline.Config.Samplerhook
 func (s *TraceSampler) Hook(fields map[string]interface{}) (sample bool, rate int) {
+	// Always sample in spans that have been set to do so via the `SetSpanSampledIn` function
+	if v, ok := fields["meta.keep.span"]; ok {
+		if keep, ok := v.(bool); ok && keep {
+			return true, 1
+		}
+	}
+
 	key := s.KeyFunc(fields)
 	rate = s.Sampler.GetSampleRate(key)
 	if shouldSample(fmt.Sprintf("%v", fields["trace.trace_id"]), rate) {

o11y/honeycomb/sampler_test.go

@@ -35,6 +35,17 @@ var samplerTests = []struct {
 		},
 		false, 0,
 	},
+	{
+		"sampled in via meta field",
+		map[string]interface{}{
+			"trace.trace_id":       "ede23f67-2048-491b-ba71-749a8a00444f",
+			"app.server_name":      "admin",
+			"request.path":         "/ready",
+			"response.status_code": 200,
+			"meta.keep.span":       true,
+		},
+		true, 1,
+	},
 	{
 		"ready-check with no problems but trace hits sample rate",
 		map[string]interface{}{

o11y/otel/sampler.go

@@ -19,7 +19,7 @@ func (s deterministicSampler) shouldSample(p sdktrace.ReadOnlySpan) (bool, uint)
 		fields[string(attr.Key)] = attr.Value.AsInterface()
 	}
 
-	// Always sample in spans that have the `meta.keep.span` field set to `true`
+	// Always sample in spans that have been set to do so via the `SetSpanSampledIn` function
 	if v, ok := fields["meta.keep.span"]; ok {
 		if keep, ok := v.(bool); ok && keep {
 			return true, 1