Logical replication details with pg_subscription_rel

Proposal

Use case. Why is this important?

We use logical replication and already have slot replication monitoring. However, this doesn't give a precise view: pg_subscription_rel gives useful information about logical replication health:

SELECT *, srrelid::regclass FROM pg_subscription_rel;

 srsubid | srrelid | srsubstate |   srsublsn    |                srrelid                 
---------+---------+------------+---------------+----------------------------------------
  271645 |   16812 | d          |               | schema1.tableA
  271645 |   17663 | d          |               | schema2.tableB
  271645 |   25779 | r          | B5C4/DB99BA58 | schema1.tableC
  271645 |   16800 | r          | B5C4/DB99BA58 | schema2.tableD
[...]

This could be useful to get metrics like

pg_subscription_rel{relation="schema1.tableA", state="d"}
pg_subscription_rel{relation="schema2.tableB", state="d"}
pg_subscription_rel{relation="schema1.tableC", state="r"}
pg_subscription_rel{relation="schema2.tableD", state="r"}

I'm not sure how lsn data could be useful here but it's still a good start to me.

If the format looks ok, we can provide a PR for this :)

No file or test is named. Start by reviewing the existing slot replication monitoring and the PostgreSQL pg_subscription_rel documentation; the work is done when logical replication relation state is exposed as useful Prometheus metrics, with the proposed relation and state labels represented consistently.