Export safe_wal_size and wal_status to monitor for lost replication slots when using max_slot_wal_keep_size

[MarcWort]

Proposal

We set the option max_slot_wal_keep_size supported by postgres >= 13 and want to alert if the slot is lost. I cannot find a reliable solution to do this except using wal_status and safe_wal_size which is currently not exported.

documentation: https://www.postgresql.org/docs/13/view-pg-replication-slots.html

example:

postgres=# SELECT * FROM pg_catalog.pg_replication_slots;
 slot_name | plugin | slot_type | datoid | database | temporary | active | active_pid | xmin | catalog_xmin | restart_lsn | confirmed_flush_lsn | wal_status | safe_wal_size
-----------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------+------------+---------------
 barman    |        | physical  |        |          | f         | f      |            |      |              | 0/B0000000  |                     | unreserved |    -323906992
(1 rows)

postgres=# SELECT * FROM pg_catalog.pg_replication_slots;
 slot_name | plugin | slot_type | datoid | database | temporary | active | active_pid | xmin | catalog_xmin | restart_lsn | confirmed_flush_lsn | wal_status | safe_wal_size
-----------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------+------------+---------------
 barman    |        | physical  |        |          | f         | f      |            |      |              |             |                     | lost       |
(1 rows)