Fix sampling rate (#180)

Author: nhulston

internal/extension/extension.go

@@ -164,8 +164,17 @@ func (em *ExtensionManager) SendEndInvocationRequest(ctx context.Context, functi
 			req.Header.Set(string(DdSamplingPriority), samplingPriority)
 		}
 	} else {
-		req.Header.Set(string(DdTraceId), fmt.Sprint(functionExecutionSpan.Context().TraceID()))
-		req.Header.Set(string(DdSpanId), fmt.Sprint(functionExecutionSpan.Context().SpanID()))
+		spanContext := functionExecutionSpan.Context()
+		req.Header.Set(string(DdTraceId), fmt.Sprint(spanContext.TraceID()))
+		req.Header.Set(string(DdSpanId), fmt.Sprint(spanContext.SpanID()))
+
+		// Try to get sampling priority
+		// Check if the context implements SamplingPriority method
+		if pc, ok := spanContext.(interface{ SamplingPriority() (int, bool) }); ok && pc != nil {
+			if priority, ok := pc.SamplingPriority(); ok {
+				req.Header.Set(string(DdSamplingPriority), fmt.Sprint(priority))
+			}
+		}
 	}
 
 	resp, err := em.httpClient.Do(req)

internal/extension/extension_test.go

@@ -223,6 +223,34 @@ func TestExtensionEndInvocationError(t *testing.T) {
 	assert.Contains(t, logOutput, "could not send end invocation payload to the extension")
 }
 
+type mockSpanContext struct {
+	ddtrace.SpanContext
+}
+
+func (m mockSpanContext) TraceID() uint64               { return 123 }
+func (m mockSpanContext) SpanID() uint64                { return 456 }
+func (m mockSpanContext) SamplingPriority() (int, bool) { return -1, true }
+
+type mockSpan struct{ ddtrace.Span }
+
+func (m mockSpan) Context() ddtrace.SpanContext { return mockSpanContext{} }
+
+func TestExtensionEndInvocationSamplingPriority(t *testing.T) {
+	headers := http.Header{}
+	em := &ExtensionManager{httpClient: capturingClient{hdr: headers}}
+	span := &mockSpan{}
+
+	// When priority in context, use that value
+	ctx := context.WithValue(context.Background(), DdTraceId, "123")
+	ctx = context.WithValue(ctx, DdSamplingPriority, "2")
+	em.SendEndInvocationRequest(ctx, span, ddtrace.FinishConfig{})
+	assert.Equal(t, "2", headers.Get("X-Datadog-Sampling-Priority"))
+
+	// When no context, get priority from span
+	em.SendEndInvocationRequest(context.Background(), span, ddtrace.FinishConfig{})
+	assert.Equal(t, "-1", headers.Get("X-Datadog-Sampling-Priority"))
+}
+
 type capturingClient struct {
 	hdr http.Header
 }