Gitlab Collector: Xid collector and test

[Sticksman]

Split from #819

Will need to merge collector_test with #841

[danzika]

Hello @Sticksman, thanks for this PR, it addresses exactly what I was looking for. May I just kindly suggest the SQL enhancement so that it provides somewhat more thorough insight?

SELECT v.current_xid, v.committed_max_age, (2147483647 - v.committed_max_age) % (2^52)::bigint AS committed_xids_until_wraparound, CASE WHEN pg_is_in_recovery() THEN NULL::bigint ELSE txid_snapshot_xmin(txid_current_snapshot()) % (2^52)::bigint END AS uncommitted_max_age, CASE WHEN pg_is_in_recovery() THEN NULL::bigint ELSE (2147483647 - (txid_current() - txid_snapshot_xmin(txid_current_snapshot()))) END uncommitted_xids_until_wraparound FROM ( SELECT CASE WHEN pg_is_in_recovery() THEN NULL::bigint ELSE txid_current() % (2^52)::bigint END AS current_xid, --current xid CASE WHEN pg_is_in_recovery() THEN NULL::bigint ELSE MAX(age(c.relfrozenxid))::bigint END as committed_max_age FROM pg_class c JOIN pg_namespace n ON c.relnamespace = n.oid WHERE c.relkind = 'r' AND n.nspname NOT IN ('pg_catalog', 'information_schema', 'pg_toast') ) v;

With these metrics, it would address both committed and uncommitted transactions and also provide "how far from wraparound" the DB is, directly usable by DBAs.
And extrapolating the current_xid trend, it could also help to estimate how much time there is till wraparound at "current" transaction rate.

Hope this helps, kind regards!