feat(utilization): monitor postgres wraparound time

Epic gitlab-com/gl-infra&474 (closed)

Part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13433.

This adds a long-term utilization metric (which will be included in the Tamland forecast) for Postgres wraparound time.

cc @ahmadsherif @msmiley @NikolayS @abrandl