Alert when GitLab Pages `.update` file has not been updated in a while

The GitLab Pages .update file needs to be updated frequently for the gitlab-pages daemon to refresh properly. There have been dozen of issues filed against this bug. I think we want to check that the file has been refreshed at least once every 24 hours and alert if it has not.

We should also add metrics to GitLab to report when a refresh should have happened. That way we can perhaps be a bit smarter about alerting.

Thoughts on this, @ayufan, @maratkalibek?

@stanhu With this MR we will know when the last update happened: https://gitlab.com/gitlab-org/gitlab-pages/merge_requests/11. So we could also alert when the diff between different workers is too big or when it didn't happen recently enough.

closed

Closing because of existing prometheus monitoring for pages.

added monitoring label