Track the amount of time waiting due to cost_delay

Hi hackers,

During the last pgconf.dev I attended Robert’s presentation about autovacuum and
it made me remember of an idea I had some time ago: $SUBJECT

Please find attached a patch doing so by adding a new field (aka "time_delayed")
to the pg_stat_progress_vacuum view. 

Currently one can change [autovacuum_]vacuum_cost_delay and
[auto vacuum]vacuum_cost_limit but has no reliable way to measure the impact of
the changes on the vacuum duration: one could observe the vacuum duration
variation but the correlation to the changes is not accurate (as many others
factors could impact the vacuum duration (load on the system, i/o latency,...)).

This new field reports the time that the vacuum has to sleep due to cost delay:
it could be useful to 1) measure the impact of the current cost_delay and
cost_limit settings and 2) when experimenting new values (and then help for
decision making for those parameters).

The patch is relatively small thanks to the work that has been done in
f1889729dd (to allow parallel worker to report to the leader).

[1]: https://www.pgevents.ca/events/pgconfdev2024/schedule/session/29-how-autovacuum-goes-wrong-and-can-we-please-make-it-stop-doing-that/

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com