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 -- [[ https://wikimedia.biterg.io/app/kibana#/dashboard/Gerrit-Backlog | "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