Question about monitoring PostgreSQL 10 streaming replication health

*Hello PostgreSQL community,*

I am monitoring a PostgreSQL 10 standby server using:

pg_last_xlog_receive_location()
and
pg_last_xlog_replay_location()

The current query only checks replay lag. I would like guidance on a more
comprehensive monitoring approach that can detect:

1. Standby disconnected from the primary.
2. WAL receiver process stopped.
3. Required WAL segment removed from the primary
(for example: "requested WAL segment has already been removed").

I would like to know:
- Which SQL checks are recommended for these scenarios?
- Which conditions require external monitoring or log analysis?
- What is the recommended practice for monitoring PostgreSQL 10 streaming
replication health?

Thank you.

Bilal Abdulkadir Muhammed
bilalabdulkadir286(at)gmail(dot)com