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 <[email protected]>
Reviewed-by: shveta malik <[email protected]>
Reviewed-by: Hayato Kuroda <[email protected]>
Reviewed-by: Ashutosh Sharma <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Discussion: https://postgr.es/m/CAE9k0PkhfKrTEAsGz4DjOhEj1nQ+hbQVfvWUxNacD38ibW3a1g@mail.gmail.com