Add prometheus query to gitaly permission denied tips

Jacob Vosmaerrequested to merge
jv-gitaly-authentication-errors into master

Follow-up to !1586 (merged)

Closes https://gitlab.com/gitlab-org/gitaly/issues/2090.

We recently added a prometheus counter to Gitaly that tracks why we returned PermissionDenied. This change documents that that counter is there.

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