pgsql: Add slotsync skip statistics.

Add slotsync skip statistics.

This patch adds two new columns to the pg_stat_replication_slots view:
slotsync_skip_count - the total number of times a slotsync operation was
skipped.
slotsync_skip_at - the timestamp of the most recent skip.

These additions provide better visibility into replication slot
synchronization behavior.

A future patch will introduce the slotsync_skip_reason column in
pg_replication_slots to capture the reason for skip.

Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAE9k0PkhfKrTEAsGz4DjOhEj1nQ+hbQVfvWUxNacD38ibW3a1g@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/76b78721ca49c16dfcbfbd2f4f87fcdb2db2bbb6

Modified Files
--------------
contrib/test_decoding/expected/stats.out           | 12 +--
doc/src/sgml/monitoring.sgml                       | 24 ++++++
src/backend/catalog/system_views.sql               |  2 +
src/backend/replication/logical/slotsync.c         | 89 ++++++++++++++++------
src/backend/utils/activity/pgstat_replslot.c       | 32 +++++++-
src/backend/utils/adt/pgstatfuncs.c                | 18 ++++-
src/include/catalog/catversion.h                   |  2 +-
src/include/catalog/pg_proc.dat                    |  6 +-
src/include/pgstat.h                               |  5 +-
.../recovery/t/040_standby_failover_slots_sync.pl  | 60 ++++++++++++++-
src/test/regress/expected/rules.out                |  4 +-
11 files changed, 212 insertions(+), 42 deletions(-)