fix(ddtrace): ensure sampling is propagated on OpenTelemetry

ddtrace/opentelemetry/tracer.go

@@ -165,3 +165,7 @@ func (c *otelCtxToDDCtx) SpanID() uint64 {
 }
 
 func (c *otelCtxToDDCtx) ForeachBaggageItem(_ func(k, v string) bool) {}
+
+func (c *otelCtxToDDCtx) IsSampled() bool {
+	return c.oc.IsSampled()
+}

ddtrace/opentelemetry/tracer_test.go

@@ -398,3 +398,28 @@ func TestMergeOtelDDBaggage(t *testing.T) {
 		assert.Equal("otelValue", value)
 	})
 }
+
+func TestSamplingDecision(t *testing.T) {
+	assert := assert.New(t)
+	tp := NewTracerProvider(
+		tracer.WithSamplingRules([]tracer.SamplingRule{
+			{Rate: 0}, // This should be applied only when a brand new root span is started and should be ignored for a non-root span
+		}),
+	)
+	defer tp.Shutdown()
+	otel.SetTracerProvider(tp)
+	tr := otel.Tracer("")
+
+	parentSpanContext := oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
+		TraceID:    oteltrace.TraceID{0xAA},
+		SpanID:     oteltrace.SpanID{0x01},
+		TraceFlags: oteltrace.FlagsSampled, // the parent span is sampled, so its child spans should be sampled too
+	})
+	ctx := oteltrace.ContextWithSpanContext(context.Background(), parentSpanContext)
+	_, span := tr.Start(ctx, "test")
+	span.End()
+
+	childSpanContext := span.SpanContext()
+	assert.Equal(parentSpanContext.TraceID(), childSpanContext.TraceID())
+	assert.True(childSpanContext.IsSampled(), "parent span is sampled, but child span is not sampled")
+}

ddtrace/tracer/spancontext.go

@@ -126,6 +126,10 @@
 	Tags() map[string]string
 }
 
+type otelContextAdapter interface {
+	IsSampled() bool
+}
+
 // FromGenericCtx converts a ddtrace.SpanContext to a *SpanContext, which can be used
 // to start child spans.
 func FromGenericCtx(c ddtrace.SpanContext) *SpanContext {
@@ -138,16 +142,21 @@
 		sc.baggage[k] = v
 		return true
 	})
-	ctx, ok := c.(spanContextV1Adapter)
-	if !ok {
-		return &sc
-	}
-	sc.origin = ctx.Origin()
-	sc.trace = newTrace()
-	sc.trace.priority = ctx.Priority()
-	sc.trace.samplingDecision = samplingDecision(ctx.SamplingDecision())
-	sc.trace.tags = ctx.Tags()
-	sc.trace.propagatingTags = ctx.PropagatingTags()
+	switch ctx := c.(type) {
+	case spanContextV1Adapter:
+		sc.origin = ctx.Origin()
+		sc.trace = newTrace()
+		sc.trace.priority = ctx.Priority()
+		sc.trace.samplingDecision = samplingDecision(ctx.SamplingDecision())
+		sc.trace.tags = ctx.Tags()
+		sc.trace.propagatingTags = ctx.PropagatingTags()
+	case otelContextAdapter:
+		sc.trace = newTrace()
+		if ctx.IsSampled() {
+			sc.setSamplingPriority(1, samplernames.Default)
+			sc.trace.keep()
+		}
+	}
 	return &sc
 }
 
@@ -252,7 +261,7 @@
 	if c == nil {
 		return
 	}
 	if atomic.LoadUint32(&c.hasBaggage) == 0 {
 		return
 	}
 	c.mu.RLock()