Metrics provider for deployments and services behind Envoy

Assumes `envoy:smi` as the mesh provider name as I've successfully tested the progressive delivery for Envoy + Crossover with it.

This enhances Flagger to translate it to the metrics provider name of `envoy` for deployment targets, or `envoy:service` for service targets.

The `envoy` metrics provider is equivalent to `appmesh`, as both relies on the same set of standard metrics exposed by Envoy itself.

The `envoy:service` is almost the same as the `envoy` provider, but removing the condition on pod name, as we only need to filter on the backing service name = envoy_cluster_name. We don't consider other Envoy xDS implementations that uses anything that is different to original servicen ames as `envoy_cluster_name`, for now.

Ref #385

cmd/flagger/main.go

@@ -161,7 +161,7 @@ func main() {
 		logger.Infof("Watching namespace %s", namespace)
 	}
 
-	observerFactory, err := metrics.NewFactory(metricsServer, meshProvider, 5*time.Second)
+	observerFactory, err := metrics.NewFactory(metricsServer, 5*time.Second)
 	if err != nil {
 		logger.Fatalf("Error building prometheus client: %s", err.Error())
 	}