Prometheus /metric endpoint to expose Ambari Metrics Service for Prometheus to use as a scrape target

Add Prometheus metrics integration by exposing all metrics via a new /metrics endpoint in Prometheus expected format.

https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config

Prometheus metrics format for that endpoint:

https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md