Add DB row count metrics for free plan
In https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/163 we added row count queries for the Rails DB that allow us to track the progress of the .com registry migration Phase 2.
What we're lacking now is a way to track intra-plan progress, i.e., be able to tell how many free, premium or ultimate repositories have been migrated already.
This will be possible once #356218 (closed) is complete. As such, we should start by adding row count queries specifically for the free plan.
Edited by João Pereira