pgsql: Add new pgstats routine to split pending data setup

Add new pgstats routine to split pending data setup

This commit adds pgstat_prep_pending_from_entry_ref(), a new pgstats
routine that is able to prepare an existing PgStat_EntryRef to receive
pending stats. This split gives a way for callers to obtain first a
reference via pgstat_get_entry_ref(), then set up pending data as two
separate, distinctive, steps.

Previously, the only way to get an entry reference with pending data
ready was pgstat_prep_pending_entry(), which bundles lookup, creation,
and pending setup in a single call. Callers that need finer control
over the entry creation had no way to attach pending data to an
already-obtained entry reference. One case where this has shown to
matter for a stats kind is where one wants to check some capacity (for
example where a GUC bounds the maximum numer of entries allowed) before
deciding if a new entry should be created. So this split can help in
reducing calls to pgstat_get_entry_ref(), meaning less shmem hash table
lookups.

The only logical ordering change is that pgStatPendingContext is
initialized after calling pgstat_get_entry_ref() in
pgstat_prep_pending_entry(). This does not matter in practice.

pgstat_prep_pending_entry() is refactored to use the new function
internally. All the existing callers are unchanged. Existing
out-of-core custom stats kinds should see no impact.

Author: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAA5RZ0sV+TsLejUMhAM=PJoOm8u-t8ru7B67KvyLCy=19sM87g@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e65c331b8fbf8c9632b62c5a9dcb589cbd3046a8

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 55 +++++++++++++++++++++++--------------
src/include/utils/pgstat_internal.h | 1 +
2 files changed, 36 insertions(+), 20 deletions(-)