Adds mirror monitoring query

This MR aims to add more monitoring to the mirroring feature to help us figure out what to improve.

\cc @stanhu @pcarranza @DouweM

The query that I used to write this one was the one generated by Project.mirrors_to_sync(Time.now) (https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/ee/project.rb#L43).

We should probably add some more queries. For example, checking how many mirrors have run in a 12 hour period.

assigned to @ahmadsherif

Just a comment and a Rubocop offense :).

assigned to @tiagonbotelho

assigned to @ahmadsherif

unmarked as a Work In Progress

resolved all discussions

@stanhu if we start needing more queries I will add them as well in a new MR thanks for the suggestion!

approved this merge request

merged

@tiagonbotelho Thanks!