Implement a metric for RPC requests with labels on the identity, so that
administrators can monitor the source of requests within the cluster. This
changeset demonstrates the change with the new `ACL.WhoAmI` RPC, and we'll wire
up the remaining RPCs once we've threaded the new pre-forwarding authentication
through the all.
Note that metrics are measured after we forward but before we return any
authentication error. This ensures that we only emit metrics on the server that
actually serves the request. We'll perform rate limiting at the same place.
Includes telemetry configuration to omit identity labels.
