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(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz(at)ip-10-97-1-34(dot)eu-west-3(dot)compute(dot)internal

Branch
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master

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

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