pkg/tracer: added single span sampling at trace finish

ddtrace/tracer/single_sampler.go

@@ -6,11 +6,13 @@
 package tracer
 
 import (
+	"math"
+	"time"
+
 	"golang.org/x/time/rate"
+
 	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace/ext"
 	"gopkg.in/DataDog/dd-trace-go.v1/internal/samplernames"
-	"math"
-	"time"
 )
 
 // singleSpanRulesSampler allows a user-defined list of rules to apply to spans
@@ -54,7 +56,7 @@ func (rs *singleSpanRulesSampler) apply(span *span) bool {
 }
 
 func (rs *singleSpanRulesSampler) applyRate(span *span, rule SamplingRule, rate float64, now time.Time) {
-	span.SetTag(keyRulesSamplerAppliedRate, rate)
+	span.setMetric(keyRulesSamplerAppliedRate, rate)
 	if !sampledByRate(span.SpanID, rate) {
 		span.setSamplingPriority(ext.PriorityUserReject, samplernames.RuleRate, rate)
 		return

ddtrace/tracer/spancontext.go

@@ -304,6 +304,9 @@ func (t *trace) finishedOne(s *span) {
 	if !ok {
 		return
 	}
+	for i := range t.spans {
+		tr.singleSpanRulesSampling.apply(t.spans[i])
+	}
 	// we have a tracer that can receive completed traces.
 	atomic.AddInt64(&tr.spansFinished, int64(len(t.spans)))
 	sd := samplingDecision(atomic.LoadInt64((*int64)(&t.samplingDecision)))

ddtrace/tracer/tracer_test.go

@@ -367,6 +367,28 @@ func TestSamplingDecision(t *testing.T) {
 		assert.Equal(t, "", span.context.trace.tags[keyUpstreamServices])
 		assert.Equal(t, decisionDrop, span.context.trace.samplingDecision)
 	})
+	t.Run("client_dropped", func(t *testing.T) {
+		os.Setenv("DD_SPAN_SAMPLING_RULES", `[{"service": "test_*","name":"*_1", "sample_rate": 1.0, "max_per_second": 15.0}]`)
+		defer os.Unsetenv("DD_SPAN_SAMPLING_RULES")
+		tracer, _, _, stop := startTestTracer(t)
+		defer stop()
+		tracer.config.agent.DropP0s = true
+		tracer.config.sampler = NewRateSampler(0)
+		tracer.prioritySampling.defaultRate = 0
+		tracer.config.serviceName = "test_service"
+		span := tracer.StartSpan("name_1").(*span)
+		child := tracer.StartSpan("name_2", ChildOf(span.context))
+		child.Finish()
+		span.Finish()
+		assert.Equal(t, float64(ext.PriorityAutoReject), span.Metrics[keySamplingPriority])
+		// this trace won't be sent to the agent,
+		// therefore not necessary to populate keyUpstreamServices
+		assert.Equal(t, "", span.context.trace.tags[keyUpstreamServices])
+		assert.Equal(t, decisionDrop, span.context.trace.samplingDecision)
+		assert.Equal(t, 8.0, span.Metrics[ext.SpanSamplingMechanism])
+		assert.Equal(t, 1.0, span.Metrics[ext.SingleSpanSamplingRuleRate])
+		assert.Equal(t, 15.0, span.Metrics[ext.SingleSpanSamplingMPS])
+	})
 }
 
 func TestTracerRuntimeMetrics(t *testing.T) {