Document prometheus metrics support

[tamalsaha]

https://github.com/appscode/guard/blob/master/server/prometheus.go

[tamalsaha]

TODOs:

• [ ] Add installer flags to auto generate ServiceMonitor and/or Service annotations for Prometheus scrapper.
• [ ] Document the e2e setup docs like https://github.com/appscode/voyager/blob/master/docs/guides/ingress/monitoring/haproxy-stats.md