Docs how to use check_postgres to check pglogical replication

[guettli]

It would be nice if you could add a note in the docs how to use check_postgres to monitor logical replication (pglogical).

I was told that check_postgres should already support this.

[petere]

I don't think there is built-in support for monitoring pglogical. The way to monitor pglogical would be to use a custom_query check against the various views that are available. The same applies to other replication systems. (You could also use the replicate_row check, but that might not cover all scenarios that pglogical and other logical replication systems support.)