Document Native Prometheus Monitoring

As we have Prometheus metrics natively in server now and with the exporter it would be useful to create an explicit metrics service so people can use it with the Prometheus operator ServiceMonitor or more easily via `kubernetes_sd_config` rather than having to write some complex rules to strip out all the other ports from the pod.

This could still be managed by the monitoring.enabled flag.

With server 7 should we be disabling the Prometheus metrics in some way if people explicitly disable it?