feat(postgres): add TXID wraparound monitoring

cc @abrandl @msmiley @Finotto @NikolayS

• Part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9965
• Part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13392#note_581743954

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1. Rename pg_database_datfrozenxid_and_datminmxid_age_datfrozenxid and pg_database_datfrozenxid_and_datminmxid_age_datminmxid to pg_database_wraparound_age_datfrozenxid and pg_database_wraparound_age_datminmxid respectively.
   1. This rename is safe, since the old metric was not consumed in any dashboards, alerts or recording rules.
2. Switch from age(datminmxid) to mxid_age(datminmxid) to correct the age calculation

The output of the query on the pg primary presently is as follows:

       datname       | age_datfrozenxid | age_datminmxid
---------------------+------------------+----------------
 template1           |        180765323 |        2604050
 gitlabhq_production |        199895030 |        7402384
 postgres            |        182343280 |        6985407

Next step will be to update gitlab-com/runbooks!3584 (merged) to use the renamed metrics. The query in that MR is likely to remain broadly similar to what it is at present.