Set up monitoring for the LooseForeignKey feature

Example payload submitted by the cleanup worker:

{
  "over_limit": true,
  "delete_count_by_table": {
    "ci_builds": 3012,
    "ci_job_artifacts": 150034
  },
  "update_count_by_table": {
    "merge_request_metrics": 731
  },
  "delete_count": 153046,
  "update_count": 731
}

The following discussion from !69165 (merged) should be addressed:

• @ayufan started a discussion:

  Seems ideal to observe with prometheus metric:

  • scoped to table?
  • deletes/updates by table