monitor GitLab's incoming email processing
In #41409 (closed), incoming email stopped being processed by GitLab. No alarm was raised, and only because @boklm noticed did we even know we need to do something.
We should monitor the number of mails in /srv/mail/git@gitlab.torproject.org/Maildir/. If it's above zero for, say, two minutes, a flag should be raised. We should also check the age of that mailbox so that it's not older than, say, a week or so, to confirm that email is coming in as well, although this is a poor replacement for end-to-end testing...