Record metrics for invocations when they start

* This experimental patch records metrics as invocations start
so that the metrics can be used to make better scale to zero
decisions in faas-idler.

Tested with Kubernetes on a single-node cluster, metrics
reported as expected. Existing metrics still report.

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

gateway/handlers/notifier_wrapper_test.go

@@ -84,6 +84,6 @@ type testNotifier struct {
 }
 
 // Notify about service metrics
-func (tf *testNotifier) Notify(method string, URL string, originalURL string, statusCode int, duration time.Duration) {
+func (tf *testNotifier) Notify(method string, URL string, originalURL string, statusCode int, event string, duration time.Duration) {
 	tf.StatusReceived = statusCode
 }