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

einterfaces/metrics.go

@@ -48,7 +48,7 @@ type MetricsInterface interface {
 	IncrementPostsSearchCounter()
 	ObservePostsSearchDuration(elapsed float64)
 	ObserveStoreMethodDuration(method, success string, elapsed float64)
-	ObserveApiEndpointDuration(endpoint, method string, elapsed float64)
+	ObserveApiEndpointDuration(endpoint, method, statusCode string, elapsed float64)
 	IncrementPostIndexCounter()
 	IncrementUserIndexCounter()
 	IncrementChannelIndexCounter()