Enhance checkpointer restartpoint statistics

Enhance checkpointer restartpoint statistics

Bhis commit introduces enhancements to the pg_stat_checkpointer view by adding
three new columns: restartpoints_timed, restartpoints_req, and
restartpoints_done. These additions aim to improve the visibility and
monitoring of restartpoint processes on replicas.

Previously, it was challenging to differentiate between successful and failed
restartpoint requests. This limitation arises because restartpoints on replicas
are dependent on checkpoint records from the primary, and cannot occur more
frequently than these checkpoints.

The new columns allow for clear distinction and tracking of restartpoint
requests, their triggers, and successful completions.  This enhancement aids
database administrators and developers in better understanding and diagnosing
issues related to restartpoint behavior, particularly in scenarios where
restartpoint requests may fail.

System catalog is changed.  Catversion is bumped.

Discussion: https://postgr.es/m/99b2ccd1-a77a-962a-0837-191cdf56c2b9%40inbox.ru
Author: Anton A. Melnikov
Reviewed-by: Kyotaro Horiguchi, Alexander Korotkov