Update Prometheus database metrics computation after meta change

[stefan-k]

Currently the database metrics exposed via Prometheus are still based on the old Record design which had site_id, group_id and user_id. The code was obviously updated to fetch these three values from the meta information in the database, but in the long run this should be implemented in a more general way. I envision something like this: For each possible pair (key, [value]), compute how often it is present in the database. Users should be able to pick the keys of interest in the configuration.