Database decomposition: Prometheus DB metrics per database

See &6134 (closed) .

Code at https://gitlab.com/gitlab-org/gitlab/-/blob/8da3c665619d5b390ba102df9158963f0098f8ee/lib/gitlab/metrics/subscribers/active_record.rb#L47

Consider using labels for each database.

First part for counters will be handled in !66885 (merged) . The next part for adding durations should be a small refactoring on top of that.