Replace peek-pg with our own implementation

This uses an ActiveRecord subscriber to get queries and calculate the total query time from that. This means that the total will always be consistent with the queries in the table. It does however mean that we could potentially miss some queries that don't go through ActiveRecord.

Making this change also allows us to unify the response JSON a little bit, making the frontend slightly simpler as a result.

This will help with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31054 because we will have full control over the JSON emitted by all of our detailed metrics.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/44441.