Store sampling.probability in sampled span attributes

processor/samplingprocessor/probabilisticsamplerprocessor/README.md

@@ -20,6 +20,10 @@ The following configuration options can be modified:
 - `hash_seed` (no default): An integer used to compute the hash algorithm. Note that all collectors for a given tier (e.g. behind the same load balancer) should have the same hash_seed.
 - `sampling_percentage` (default = 0): Percentage at which traces are sampled; >= 100 samples all traces
 
+The sampled spans have [`sampling.probability`](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#sampling)
+attribute added, which includes the value in range of `(0, 1.0]` representing the probability with which the record
+was sampled. If the span was already sampled before and the attribute is present, the existing value is multiplied.
+
 Examples:
 
 ```yaml

processor/samplingprocessor/probabilisticsamplerprocessor/probabilisticsampler.go

@@ -22,6 +22,7 @@ import (
 	"go.opentelemetry.io/collector/component/componenterror"
 	"go.opentelemetry.io/collector/consumer"
 	"go.opentelemetry.io/collector/consumer/pdata"
+	"go.opentelemetry.io/collector/translator/conventions"
 )
 
 // samplingPriority has the semantic result of parsing the "sampling.priority"
@@ -49,9 +50,10 @@ const (
 )
 
 type tracesamplerprocessor struct {
-	nextConsumer       consumer.TracesConsumer
-	scaledSamplingRate uint32
-	hashSeed           uint32
+	nextConsumer        consumer.TracesConsumer
+	scaledSamplingRate  uint32
+	samplingProbability float64
+	hashSeed            uint32
 }
 
 // newTraceProcessor returns a processor.TracesProcessor that will perform head sampling according to the given
@@ -64,8 +66,9 @@ func newTraceProcessor(nextConsumer consumer.TracesConsumer, cfg Config) (compon
 	return &tracesamplerprocessor{
 		nextConsumer: nextConsumer,
 		// Adjust sampling percentage on private so recalculations are avoided.
-		scaledSamplingRate: uint32(cfg.SamplingPercentage * percentageScaleFactor),
-		hashSeed:           cfg.HashSeed,
+		scaledSamplingRate:  uint32(cfg.SamplingPercentage * percentageScaleFactor),
+		samplingProbability: float64(cfg.SamplingPercentage) * 0.01,
+		hashSeed:            cfg.HashSeed,
 	}, nil
 }
 
@@ -82,6 +85,16 @@ func (tsp *tracesamplerprocessor) ConsumeTraces(ctx context.Context, td pdata.Tr
 	return tsp.nextConsumer.ConsumeTraces(ctx, sampledTraceData)
 }
 
+func (tsp *tracesamplerprocessor) updateSamplingProbability(sampledSpanAttributes pdata.AttributeMap) {
+	samplingProbability := tsp.samplingProbability
+	attr, found := sampledSpanAttributes.Get(conventions.AttributeSamplingProbability)
+	if found && attr.Type() == pdata.AttributeValueDOUBLE {
+		samplingProbability *= attr.DoubleVal()
+	}
+
+	sampledSpanAttributes.UpsertDouble(conventions.AttributeSamplingProbability, samplingProbability)
+}
+
 func (tsp *tracesamplerprocessor) processTraces(resourceSpans pdata.ResourceSpans, sampledTraceData pdata.Traces) {
 	scaledSamplingRate := tsp.scaledSamplingRate
 
@@ -115,6 +128,7 @@ func (tsp *tracesamplerprocessor) processTraces(resourceSpans pdata.ResourceSpan
 				hash(tidBytes[:], tsp.hashSeed)&bitMaskHashBuckets < scaledSamplingRate
 
 			if sampled {
+				tsp.updateSamplingProbability(span.Attributes())
 				spns.Append(span)
 			}
 		}

processor/samplingprocessor/probabilisticsamplerprocessor/probabilisticsampler_test.go

@@ -28,6 +28,7 @@ import (
 	"go.opentelemetry.io/collector/consumer"
 	"go.opentelemetry.io/collector/consumer/consumertest"
 	"go.opentelemetry.io/collector/consumer/pdata"
+	"go.opentelemetry.io/collector/translator/conventions"
 	tracetranslator "go.opentelemetry.io/collector/translator/trace"
 )
 
@@ -71,6 +72,7 @@ func TestNewTraceProcessor(t *testing.T) {
 			if !tt.wantErr {
 				// The truncation below with uint32 cannot be defined at initialization (compiler error), performing it at runtime.
 				tt.want.(*tracesamplerprocessor).scaledSamplingRate = uint32(tt.cfg.SamplingPercentage * percentageScaleFactor)
+				tt.want.(*tracesamplerprocessor).samplingProbability = float64(tt.cfg.SamplingPercentage) * 0.01
 			}
 			got, err := newTraceProcessor(tt.nextConsumer, tt.cfg)
 			if (err != nil) != tt.wantErr {
@@ -227,15 +229,7 @@ func Test_tracesamplerprocessor_SamplingPercentageRange_MultipleResourceSpans(t
 
 // Test_tracesamplerprocessor_SpanSamplingPriority checks if handling of "sampling.priority" is correct.
 func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
-	singleSpanWithAttrib := func(key string, attribValue pdata.AttributeValue) pdata.Traces {
-		traces := pdata.NewTraces()
-		traces.ResourceSpans().Resize(1)
-		rs := traces.ResourceSpans().At(0)
-		rs.InstrumentationLibrarySpans().Resize(1)
-		instrLibrarySpans := rs.InstrumentationLibrarySpans().At(0)
-		instrLibrarySpans.Spans().Append(getSpanWithAttributes(key, attribValue))
-		return traces
-	}
+
 	tests := []struct {
 		name    string
 		cfg     Config
@@ -247,7 +241,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 0.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueInt(2)),
 			sampled: true,
@@ -257,7 +251,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 0.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueDouble(1)),
 			sampled: true,
@@ -267,7 +261,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 0.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueString("1")),
 			sampled: true,
@@ -277,7 +271,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 100.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueInt(0)),
 		},
@@ -286,7 +280,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 100.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueDouble(0)),
 		},
@@ -295,7 +289,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 100.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"sampling.priority",
 				pdata.NewAttributeValueString("0")),
 		},
@@ -304,7 +298,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 0.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"no.sampling.priority",
 				pdata.NewAttributeValueInt(2)),
 		},
@@ -313,7 +307,7 @@ func Test_tracesamplerprocessor_SpanSamplingPriority(t *testing.T) {
 			cfg: Config{
 				SamplingPercentage: 100.0,
 			},
-			td: singleSpanWithAttrib(
+			td: getTracesWithSpanWithAttribute(
 				"no.sampling.priority",
 				pdata.NewAttributeValueInt(2)),
 			sampled: true,
@@ -416,6 +410,59 @@ func Test_parseSpanSamplingPriority(t *testing.T) {
 	}
 }
 
+// Test_tracesamplerprocessor_SamplingProbabilityAttribute verifies if the attribute describing current sampling rate is included in sampled spans
+func Test_tracesamplerprocessor_SamplingProbabilityAttribute(t *testing.T) {
+	cfg := Config{
+		SamplingPercentage: 100.0,
+	}
+
+	tests := []struct {
+		name                             string
+		traces                           pdata.Traces
+		wantSamplingProbabilityAttribute pdata.AttributeValue
+	}{
+		{
+			name:                             "simple_span",
+			traces:                           getTracesWithSpanWithAttribute("foo", pdata.NewAttributeValueString("bar")),
+			wantSamplingProbabilityAttribute: pdata.NewAttributeValueDouble(1.0),
+		},
+		{
+			name:                             "span_came_through_sampler_already",
+			traces:                           getTracesWithSpanWithAttribute(conventions.AttributeSamplingProbability, pdata.NewAttributeValueDouble(0.01)),
+			wantSamplingProbabilityAttribute: pdata.NewAttributeValueDouble(0.01),
+		},
+		{
+			name:                             "simple_with_invalid_attribute_value",
+			traces:                           getTracesWithSpanWithAttribute(conventions.AttributeSamplingProbability, pdata.NewAttributeValueString("bar")),
+			wantSamplingProbabilityAttribute: pdata.NewAttributeValueDouble(1.0),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sink := new(consumertest.TracesSink)
+			tsp, err := newTraceProcessor(sink, cfg)
+			if err != nil {
+				t.Errorf("error when creating tracesamplerprocessor: %v", err)
+				return
+			}
+
+			if err := tsp.ConsumeTraces(context.Background(), tt.traces); err != nil {
+				t.Errorf("tracesamplerprocessor.ConsumeTraceData() error = %v", err)
+				return
+			}
+			assert.Equal(t, 1, sink.SpansCount())
+			for _, td := range sink.AllTraces() {
+				span := td.ResourceSpans().At(0).InstrumentationLibrarySpans().At(0).Spans().At(0)
+				attrValue, found := span.Attributes().Get(conventions.AttributeSamplingProbability)
+				assert.True(t, found, "Sampling probability attribute not found")
+				assert.Equal(t, tt.wantSamplingProbabilityAttribute, attrValue)
+			}
+			sink.Reset()
+		})
+	}
+}
+
 func getSpanWithAttributes(key string, value pdata.AttributeValue) pdata.Span {
 	span := pdata.NewSpan()
 	span.InitEmpty()
@@ -424,6 +471,16 @@ func getSpanWithAttributes(key string, value pdata.AttributeValue) pdata.Span {
 	return span
 }
 
+func getTracesWithSpanWithAttribute(key string, attribValue pdata.AttributeValue) pdata.Traces {
+	traces := pdata.NewTraces()
+	traces.ResourceSpans().Resize(1)
+	rs := traces.ResourceSpans().At(0)
+	rs.InstrumentationLibrarySpans().Resize(1)
+	instrLibrarySpans := rs.InstrumentationLibrarySpans().At(0)
+	instrLibrarySpans.Spans().Append(getSpanWithAttributes(key, attribValue))
+	return traces
+}
+
 // Test_hash ensures that the hash function supports different key lengths even if in
 // practice it is only expected to receive keys with length 16 (trace id length in OC proto).
 func Test_hash(t *testing.T) {

translator/conventions/opentelemetry.go

@@ -61,6 +61,7 @@ const (
 	AttributeProcessExecutablePath = "process.executable.path"
 	AttributeProcessID             = "process.pid"
 	AttributeProcessOwner          = "process.owner"
+	AttributeSamplingProbability   = "sampling.probability"
 	AttributeServiceInstance       = "service.instance.id"
 	AttributeServiceName           = "service.name"
 	AttributeServiceNamespace      = "service.namespace"