Add mode column to pg_stat_progress_vacuum

Hi hackers,

I would like to propose a patch that enhances the
pg_stat_progress_vacuum view by adding a mode column. The patch is
attached.

Although it is possible to identify an anti-wraparound VACUUM through
the process title (to prevent wraparound) or specific log entries, it
would be significantly more convenient for monitoring purposes to have
this status clearly indicated in the pg_stat_progress_vacuum view.
This would enable DBAs to immediately understand the urgency of the
vacuum process without needing to check separate logs or system
processes.

This patch introduces a mode column to provide this visibility. The
possible values are:
- normal: A standard, user-initiated VACUUM or a regular autovacuum run.
- anti-wraparound: An autovacuum run launched specifically to prevent
transaction ID wraparound.
- failsafe: A vacuum that has entered failsafe mode to prevent
imminent transaction ID wraparound.

This will allow administrators to better understand the context and
urgency of vacuum operations, which is crucial for monitoring and
troubleshooting.

Design Considerations:
When defining the scope of the anti-wraparound mode, I considered
including manual commands like VACUUM (FREEZE) or VACUUM
(DISABLE_PAGE_SKIPPING). However, I decided against this to keep the
meaning of the mode clear and simple. These options can be used for
various purposes, and overloading the anti-wraparound mode with too
many meanings could become confusing. Therefore, the current
implementation limits this mode to autovacuum runs that are explicitly
launched for wraparound prevention.

Regarding Testing:
I was able to manually verify the failsafe mode's behavior by using
the existing test script at
src/test/modules/xid_wraparound/t/001_emergency_vacuum.pl. This script
successfully triggered the failsafe condition and the view reported
the correct mode. However, I found this test to be somewhat flaky in
my environment and decided not to add it to the patch to avoid
introducing a potentially unstable test into the tree.

Thought?

--
Best regards,
Shinya Kato
NTT OSS Center