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.