chore: remove custom logic to measure size of online GC queues

Context

In !588 (merged) we introduced an application-side Prometheus metric to expose the row count of the online GC tables (which act as queues for pending tasks).

This was useful for testing in pre/gstg but we quickly realized that it's far from ideal.

Problem

Doing this measuring on the application side means running this query on every instance. This is a waste of resources for a large cluster as every instance will query for the same thing. Although we run these interleaved with a randomized jitter and in a 10m cadence to ensure we don't put too much stress on the DB, it's completely unnecessary to do so. Plus, we have multiple instances reporting different row counts, which is far from accurate.

Solution

Revert !588 (merged) and update the registry Grafana dashboard to use the new metrics exposed directly from the database cluster once https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14127 is completed.