MM-23131 Include HTTP status code in the metrics (#14240)

* ResponseWriter wrapper to get status code

For our metrics, we need the status code returned by a request
so this wrapper includes a new method StatusCode() that includes
the desired code

* Shadow the responsewriter variable in the handlers

In order to avoid confusion to people deciding what variable to use.
I've also changed the tests to reflect this change and added a new
one that checks the Flush method works

web/response_writer_wrapper.go

@@ -0,0 +1,62 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package web
+
+import (
+	"bufio"
+	"errors"
+	"net"
+	"net/http"
+)
+
+type responseWriterWrapper struct {
+	http.ResponseWriter
+	statusCode        int
+	statusCodeWritten bool
+	hijacker          http.Hijacker
+	flusher           http.Flusher
+}
+
+func newWrappedWriter(original http.ResponseWriter) *responseWriterWrapper {
+	hijacker, _ := original.(http.Hijacker)
+	flusher, _ := original.(http.Flusher)
+	return &responseWriterWrapper{
+		ResponseWriter:    original,
+		statusCodeWritten: false,
+		hijacker:          hijacker,
+		flusher:           flusher,
+	}
+}
+
+func (rw *responseWriterWrapper) StatusCode() int {
+	return rw.statusCode
+}
+
+func (rw *responseWriterWrapper) WriteHeader(statusCode int) {
+	rw.statusCode = statusCode
+	rw.statusCodeWritten = true
+	rw.ResponseWriter.WriteHeader(statusCode)
+}
+
+func (rw *responseWriterWrapper) Write(data []byte) (int, error) {
+	if !rw.statusCodeWritten {
+		rw.statusCode = http.StatusOK
+	}
+	return rw.ResponseWriter.Write(data)
+}
+
+// Using as embedded makes the ResponseWrite be stored as interface and that way
+// it loses the access to the implementation for Hijack or Flush
+func (rw *responseWriterWrapper) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if rw.hijacker == nil {
+		return nil, nil, errors.New("Hijacker interface not supported by the wrapped ResponseWriter")
+	}
+	return rw.hijacker.Hijack()
+}
+
+func (rw *responseWriterWrapper) Flush() {
+	if rw.flusher != nil {
+		rw.flusher.Flush()
+	}
+}