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/handlers.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"reflect"
 	"runtime"
+	"strconv"
 	"strings"
 	"time"
 
@@ -79,6 +80,7 @@ type Handler struct {
 }
 
 func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w = newWrappedWriter(w)
 	now := time.Now()
 
 	requestID := model.NewId()
@@ -257,8 +259,9 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 		if r.URL.Path != model.API_URL_SUFFIX+"/websocket" {
 			elapsed := float64(time.Since(now)) / float64(time.Second)
+			statusCode := strconv.Itoa(w.(*responseWriterWrapper).StatusCode())
 			c.App.Metrics().ObserveHttpRequestDuration(elapsed)
-			c.App.Metrics().ObserveApiEndpointDuration(h.HandlerName, r.Method, elapsed)
+			c.App.Metrics().ObserveApiEndpointDuration(h.HandlerName, r.Method, statusCode, elapsed)
 		}
 	}
 }