Write tutorial - How are metrics collected by Prometheus?

This is a how-to / demo style of tutorial.

A tour of the prometheus exporters we use and what sources of information they sample.

Note that a separate set of tutorials will demo how to directly query a gitlab component's /metrics endpoint and trace those metrics into the source code. So that can be omitted or just briefly mentioned in this tutorial, and maybe link to those other tutorials in the "Learn more" / "See also" section at the end of this tutorial.