aggregate backend metrics

[Keksoj]

the configurable aggregate-backend-metrics allows to accumulate metrics with the same backend id into a single cumulated value:

# TYPE http_status_2xx counter
http_status_2xx{cluster_id="MyCluster"} 11

instead of

# TYPE http_status_2xx counter
http_status_2xx{cluster_id="MyCluster",backend_id="the-backend-to-my-app"} 6
http_status_2xx{cluster_id="MyCluster",backend_id="the-backend-to-my-app-2"} 5