Document Prometheus Consul Service Discovery

Document the new Prometheus Consul Service Discovery feature in high availability documentation.

There will likely be several components to this:

• A new page in HA docs dedicated to Prometheus (and Grafana, or it could be on its own page?). This page will discuss the overall concept, including service discovery, and configuration of the monitoring server itself.
• A snippet on each HA component page outlining how to enable consul as a client and enable the various services to listen on an interface and address accessible by Prometheus for scraping.

We need to keep things are dry as possible, but also keep things clear for the user. Be sure to call out that this feature was shipped in 12.0 (assuming omnibus-gitlab!3295 (merged) ships on time).