[Geo] Track the repository sync delay

Proposal

In the Geo environment, we can check the time to replicate Database from the  Database replication lag in the Sync status Rake task (and also pg_stat_replication table).

On the other hand, there are no way to show the time to replicate data not in the DB (for example, git repository).

We can see the number and percentage of repositories left to be synced in the gitlab-rake geo:status or the UI. Under normal circumstances, this sync only takes a very short time, but it would be better to have a mechanism to track the time spent syncing these data in order to investigate any replication issues.