This tracker is used to track the number of projects are being
scheduled. This number is used in UpdateAllMirrorsWorker to control its
rescheduling frequency. This number is also a replacement for
UpdateAllMirrorsWorker#queue_size, in an initiative to remove the
dependency to Sidekiq queues.

We made two attempts to resolve this issue, but not successfully:
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80711
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79097

Changelog: other