Explore adding Prometheus as a data source for usage ping

Prometheus is GitLab's supported monitoring platform, and it is bundled and enabled by default. Given the desire to understand high-level operational details of our self-managed customers (Apdex, Topology, etc.) - this would be the natural place to try to pull from which would have the broadest perspective.

Being able to execute Prometheus queries, and returning that content back in the form of a usage ping would be great.

There are a few things to think about:

• How to know which Prometheus server to query
• How to be transparent about the queries we are running and data we are pulling pack. Generally Prometheus doesn't have sensitive information, but it can capture systems beyond GitLab depending on how it has been configured.