fix: implement default trace-id in response header

config.go

@@ -83,8 +83,15 @@ func WithFilter(filter func(r *http.Request) bool) Option {
 }
 
 // WithTraceResponseHeaderKey is used for changing response header key that contains trace id.
-func WithTraceResponseHeaderKey(name string) Option {
+// Instead of using a fixed name as the parameter, this function accepts a function
+// that takes a string (the default trace header) and returns a string (the custom trace header).
+// If the provided function is nil, the default trace header (X-Trace-ID) will be used.
+func WithTraceResponseHeaderKey(headerKeyFunc func() string) Option {
 	return optionFunc(func(cfg *config) {
-		cfg.TraceResponseHeaderKey = name
+		if headerKeyFunc == nil {
+			cfg.TraceResponseHeaderKey = traceResponseHeaderKey // Default trace header
+		} else {
+			cfg.TraceResponseHeaderKey = headerKeyFunc()
+		}
 	})
 }

middleware.go

@@ -147,8 +147,8 @@ func (tw traceware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	)
 	defer span.End()
 
-	// put trace_id to response header
-	if span.SpanContext().HasTraceID() {
+	// put trace_id to response header only when WithTraceResponseHeaderKey is used
+	if tw.traceResponseHeaderKey != "" && span.SpanContext().HasTraceID() {
 		w.Header().Add(tw.traceResponseHeaderKey, span.SpanContext().TraceID().String())
 	}
 

test/cases/sdk_test.go

@@ -367,13 +367,16 @@ func TestSDKIntegrationWithOverrideHeaderKey(t *testing.T) {
 	provider := sdktrace.NewTracerProvider()
 	provider.RegisterSpanProcessor(sr)
 
-	customeHeaderKey := "X-Custom-Trace-ID"
+	// Define a function inline that transforms the default header name to a custom header name
+	customHeaderKeyFunc := func() string {
+		return "X-Custom-Trace-ID"
+	}
 
 	router := chi.NewRouter()
 	router.Use(otelchi.Middleware(
 		"foobar",
 		otelchi.WithTracerProvider(provider),
-		otelchi.WithTraceResponseHeaderKey(customeHeaderKey),
+		otelchi.WithTraceResponseHeaderKey(customHeaderKeyFunc),
 	))
 	router.HandleFunc("/user/{id:[0-9]+}", ok)
 	router.HandleFunc("/book/{title}", ok)
@@ -392,7 +395,7 @@ func TestSDKIntegrationWithOverrideHeaderKey(t *testing.T) {
 		attribute.String("http.target", "/user/123"),
 		attribute.String("http.route", "/user/{id:[0-9]+}"),
 	)
-	require.Equal(t, w.Header().Get(customeHeaderKey), sr.Ended()[0].SpanContext().TraceID().String())
+	require.Equal(t, w.Header().Get(customHeaderKeyFunc()), sr.Ended()[0].SpanContext().TraceID().String())
 }
 
 func assertSpan(t *testing.T, span sdktrace.ReadOnlySpan, name string, kind trace.SpanKind, attrs ...attribute.KeyValue) {