Fix bug in writing headers twice when serving metrics

cmd/run.go

@@ -352,7 +352,14 @@ var runCmd = &cobra.Command{
 						span.RecordError(err)
 						sentry.CaptureException(err)
 					}
-					next.ServeHTTP(responseWriter, request)
+					// The WriteHeader method intentionally does nothing, to prevent a bug
+					// in the merging metrics that causes the headers to be written twice,
+					// which results in an error: "http: superfluous response.WriteHeader call".
+					next.ServeHTTP(
+						&metrics.HeaderBypassResponseWriter{
+							ResponseWriter: responseWriter,
+						},
+						request)
 				}
 				return http.HandlerFunc(handler)
 			}

metrics/utils.go

@@ -0,0 +1,19 @@
+package metrics
+
+import "net/http"
+
+// HeaderBypassResponseWriter implements the http.ResponseWriter interface
+// and allows us to bypass the response header when writing to the response.
+// This is useful for merging metrics from multiple sources.
+type HeaderBypassResponseWriter struct {
+	http.ResponseWriter
+}
+
+// WriteHeader intentionally does nothing, but is required to
+// implement the http.ResponseWriter.
+func (w *HeaderBypassResponseWriter) WriteHeader(code int) {}
+
+// Write writes the data to the response.
+func (w *HeaderBypassResponseWriter) Write(data []byte) (int, error) {
+	return w.ResponseWriter.Write(data)
+}