PG 15 beta metrics

[Vonng]

https://www.postgresql.org/docs/15/release-15.html

Some new features about monitoring

• new system view pg_stat_subscription_stats
• pg_ls_logicalsnapdir(), pg_ls_logicalmapdir(), and pg_ls_replslotdir().

[mjf]

This is very nice resource for comparison between major versions: PgStats.