Enhance traceability of wal_level changes for backup management

Hi,


This thread came from another thread about wal_level [1].

Mainly from backup management tools perspective
such as pgBackRest, EDB's BART and pg_probackup,
it seems worth talking about a way comprehensively
to trace and recognize wal_level changes for various purposes and values
like necessity of invalidating old backups for example.

In the thread [1], I talk about wal_level='none' but
these kind of topic applies changing wal_level to 'minimal'
from higher level too. Accordingly, I made this topic as a new independent thread.

Currently, these backup management tools described above
work when wal_level is higher than minimal
because these use physical online backup or wal archiving
but giving any useful ideas for backup management
related to wal_level changes is welcomed.

[1] -
https://www.postgresql.org/message-id/TYAPR01MB29901EBE5A3ACCE55BA99186FE320%40TYAPR01MB2990.jpnprd01.prod.outlook.com

Best Regards,
    Takamichi Osumi