Added documentation for GitLab Metrics

This adds documentation for the metrics code I recently added.

Two notes:

• I'm referring to this feature as "GitLab Metrics" everywhere simply because the code resides in the Gitlab::Metrics namespace. A different name is fine, though I think just "Metrics" might be a bit too abstract.
• Grafana isn't covered since it's fairly straight-forward to setup (there's nothing really special we can cover). I think we should cover it once we start shipping it with Omnibus.

I assigned @axil since he's our technical writer/documentation guru, feel free to re-assign if needed.

cc @sytses @axil @dzaporozhets @JobV