Monitoring API, Plug-in System or End-Point

Description

A lot of users and customers have their own favorite monitoring tool. While GitLab is focused primarily on Prometheus, others may want to use something else to monitor GitLab.

Proposal

We should provide an monitor API, plug-in system or end-point that would allow other monitoring systems to access GitLab data.
Perhaps even a way to generically export monitoring data for parsing in another tool.