Implement Unwrap() for responseWriterDelegator

If the ResponseWriter implements any of the following methods,
the ResponseController will call them as appropriate:

Flush()
FlushError() error // alternative Flush returning an error
Hijack() (net.Conn, *bufio.ReadWriter, error)
SetReadDeadline(deadline time.Time) error
SetWriteDeadline(deadline time.Time) error
EnableFullDuplex() error

If the ResponseWriter doesn't implement the methods,
the ResponseController will call Unwrap() method until it
finds a ResponseWriter in the chain

This commit implements Unwrap() method to simply return the
wrapped ResponseWriter

prometheus/promhttp/delegator.go

@@ -76,6 +76,10 @@ func (r *responseWriterDelegator) Write(b []byte) (int, error) {
 	return n, err
 }
 
+func (r *responseWriterDelegator) Unwrap() http.ResponseWriter {
+	return r.ResponseWriter
+}
+
 type (
 	closeNotifierDelegator struct{ *responseWriterDelegator }
 	flusherDelegator       struct{ *responseWriterDelegator }

prometheus/promhttp/delegator_test.go

@@ -0,0 +1,28 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package promhttp
+
+import (
+	"net/http/httptest"
+	"testing"
+)
+
+func TestResponseWriterDelegatorUnwrap(t *testing.T) {
+	w := httptest.NewRecorder()
+	rwd := &responseWriterDelegator{ResponseWriter: w}
+
+	if rwd.Unwrap() != w {
+		t.Error("unwrapped responsewriter must equal to the original responsewriter")
+	}
+}