Allows setting -prometheus-backend-port to configure the cluster
envoy_prometheus_bind_addr points to.
Allows setting -prometheus-scrape-path to configure which path
envoy_prometheus_bind_addr exposes metrics on.
-prometheus-backend-port is used by the consul-k8s metrics merging feature, to
configure envoy_prometheus_bind_addr to point to the merged metrics
endpoint that combines Envoy and service metrics so that one set of
annotations on a Pod can scrape metrics from the service and it's Envoy
sidecar.
-prometheus-scrape-path is used to allow configurability of the path
where prometheus metrics are exposed on envoy_prometheus_bind_addr.
