Draft: Document some guidelines and tips on Prometheus metrics

What does this MR do and why?

As a developer, it's unclear what Prometheus metrics are available for measurements.

There seems to be some tribal knowledge on how to discover Prometheus metrics from various sources (GitLab Rails, Postgres, GCP, Redis, etc). Such information should be documented in a developer doc.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #410287