Followup task to T100978
Add remaining KPIs from T94578 to https://wikimedia.biterg.io/app/kibana#/dashboard/Overview once they are available as data in kibana itself:
- Median age of open changesets waiting for review -- blocked on T151557
- Number of open changesets waiting for review -- "NEW Changeset Statistics" (gerrit_main_numbes_new_changesets_statistics) on "Gerrit Backlog" dashboard. TODO: Copy widget to Overview.
- Number of new changesets submitted per month -- covered by "Changesets" (gerrit_evolutionary_changesets). Already on Overview.
- Number of code uploaders per month -- covered by "Changeset Submitters" (Change-submitters-per-month__gerrit_enrich). Already on Overview.
- Number of code reviewers per month -- blocked on T151559
- Number of code committers per month -- blocked on T151558
- Number of active users in Phabricator per month -- blocked on T138002