Document Prometheus metrics being used by Gitlab SSHD
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### Problem
Gitlab SSHD [collects](ttps://gitlab.com/gitlab-org/gitlab-shell/blob/336c4a51aa332b0cc1c68798003a1d82ab4a9cc3/internal/metrics/metrics.go#L12-30) a number of metrics; however, it's not being documented anywhere.
### Questions
- Do we document Prometheus metrics for any other service? (Workhorse, Gitaly)
- Would it be useful and important to have this documentation?
- Can it be used by admins of self-managed instances to setup additional monitoring?
issue