Remove Prometheus tracking for forwarded requests

Forwarded requests were being logged in the function metrics and
I saw this when doing some testing on auto-scaling with K8s.

This change removes the call to the Prometheus notifier for any
calls which are forwarded to the provider and retains them for the
functions.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/server.go

@@ -47,7 +47,7 @@ func main() {
 		Metrics: &metricsOptions,
 	}
 	functionNotifiers := []handlers.HTTPNotifier{loggingNotifier, prometheusNotifier}
-	forwardingNotifiers := []handlers.HTTPNotifier{loggingNotifier, prometheusNotifier}
+	forwardingNotifiers := []handlers.HTTPNotifier{loggingNotifier}
 
 	urlResolver := handlers.SingleHostBaseURLResolver{BaseURL: config.FunctionsProviderURL.String()}
 	var functionURLResolver handlers.BaseURLResolver