pgsql: Refactor code of pg_stat_get_wal() building result tuple

Refactor code of pg_stat_get_wal() building result tuple

This commit adds to pgstatfuncs.c a new routine called
pg_stat_wal_build_tuple(), helper routine for pg_stat_get_wal().  This
is in charge of filling one tuple based on the contents of
PgStat_WalStats retrieved from pgstats.

This refactoring will be used by an upcoming patch introducing
backend-level WAL statistics, simplifying the main patch.  Note that
it is not possible for stats_reset to be NULL in pg_stat_wal; backend
statistics need to be able to handle this case.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/495864a4cf16530c5174f264668c87b780c4cf22

Modified Files
--------------
src/backend/utils/adt/pgstatfuncs.c | 44 +++++++++++++++++++++++++------------
1 file changed, 30 insertions(+), 14 deletions(-)