promhttp: Ignore informational header when handling status code

Signed-off-by: Janusz Marcinkiewicz <[email protected]>

Author: VirrageS

prometheus/promhttp/delegator.go

@@ -53,15 +53,25 @@ func (r *responseWriterDelegator) Written() int64 {
 }
 
 func (r *responseWriterDelegator) WriteHeader(code int) {
-	if r.observeWriteHeader != nil && !r.wroteHeader {
-		// Only call observeWriteHeader for the 1st time. It's a bug if
-		// WriteHeader is called more than once, but we want to protect
-		// against it here. Note that we still delegate the WriteHeader
-		// to the original ResponseWriter to not mask the bug from it.
-		r.observeWriteHeader(code)
+	if !r.wroteHeader {
+		// Ignore informational headers.
+		// Based on https://github.com/golang/go/blob/go1.24.1/src/net/http/server.go#L1216
+		if code >= 100 && code <= 199 && code != http.StatusSwitchingProtocols {
+			goto propagate
+		}
+
+		if r.observeWriteHeader != nil {
+			// Only call observeWriteHeader for the 1st time. It's a bug if
+			// WriteHeader is called more than once, but we want to protect
+			// against it here. Note that we still delegate the WriteHeader
+			// to the original ResponseWriter to not mask the bug from it.
+			r.observeWriteHeader(code)
+		}
 	}
 	r.status = code
 	r.wroteHeader = true
+
+propagate:
 	r.ResponseWriter.WriteHeader(code)
 }