[RFE] Add Service object for metrics

[evgkrsk]

Please add Service object for metrics port (tcp/2381 by default) to use with things like ServiceMonitor (Prometheus).

In etcd-operator-0.4.1, there is just service and service-headless Services (tcp/2379 and tcp/2380).

[tym83]

@evgkrsk Hi! We'll discuss it on the next etcd-operator meeting, I think it will be at February, 25 and come back with information