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/mocks/MetricsInterface.go

@@ -171,9 +171,9 @@ func (_m *MetricsInterface) IncrementWebsocketEvent(eventType string) {
 	_m.Called(eventType)
 }
 
-// ObserveApiEndpointDuration provides a mock function with given fields: endpoint, method, elapsed
-func (_m *MetricsInterface) ObserveApiEndpointDuration(endpoint string, method string, elapsed float64) {
-	_m.Called(endpoint, method, elapsed)
+// ObserveApiEndpointDuration provides a mock function with given fields: endpoint, method, statusCode, elapsed
+func (_m *MetricsInterface) ObserveApiEndpointDuration(endpoint string, method string, statusCode string, elapsed float64) {
+	_m.Called(endpoint, method, statusCode, elapsed)
 }
 
 // ObserveClusterRequestDuration provides a mock function with given fields: elapsed