fix: gateway trace headers 5248 (#5256)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

gateway/internal/headerprocessor.go

@@ -11,16 +11,40 @@ const (
 	metadataPrefix       = "gateway-"
 )
 
+// OpenTelemetry trace propagation headers that need to be forwarded to gRPC metadata.
+// These headers are used by the W3C Trace Context standard for distributed tracing.
+var traceHeaders = map[string]bool{
+	"traceparent": true,
+	"tracestate":  true,
+	"baggage":     true,
+}
+
 // ProcessHeaders builds the headers for the gateway from HTTP headers.
+// It forwards both custom metadata headers (with Grpc-Metadata- prefix)
+// and OpenTelemetry trace propagation headers (traceparent, tracestate, baggage)
+// to ensure distributed tracing works correctly across the gateway.
 func ProcessHeaders(header http.Header) []string {
 	var headers []string
 
 	for k, v := range header {
+		// Forward OpenTelemetry trace propagation headers
+		// These must be lowercase per gRPC metadata conventions
+		if lowerKey := strings.ToLower(k); traceHeaders[lowerKey] {
+			for _, vv := range v {
+				headers = append(headers, lowerKey+":"+vv)
+			}
+			continue
+		}
+
+		// Forward custom metadata headers with Grpc-Metadata- prefix
 		if !strings.HasPrefix(k, metadataHeaderPrefix) {
 			continue
 		}
 
-		key := fmt.Sprintf("%s%s", metadataPrefix, strings.TrimPrefix(k, metadataHeaderPrefix))
+		// gRPC metadata keys are case-insensitive and stored as lowercase,
+		// so we lowercase the key to match gRPC conventions
+		trimmedKey := strings.TrimPrefix(k, metadataHeaderPrefix)
+		key := strings.ToLower(fmt.Sprintf("%s%s", metadataPrefix, trimmedKey))
 		for _, vv := range v {
 			headers = append(headers, key+":"+vv)
 		}