pgsql: Flush WAL stats in bgwriter

Flush WAL stats in bgwriter

bgwriter can write out WAL, but did not flush the WAL pgstat counters,
so the writes were not seen in pg_stat_wal.

Back-patch to v14, where pg_stat_wal was introduced.

Author: Nazir Bilal Yavuz
Reviewed-by: Matthias van de Meent, Kyotaro Horiguchi
Discussion:
https://www.postgresql.org/message-id/CAN55FZ2FPYngovZstr%3D3w1KSEHe6toiZwrurbhspfkXe5UDocg%40mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0684d1949fb43ec21480db627cd13a282ca41f6e

Modified Files
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src/backend/postmaster/bgwriter.c | 1 +
1 file changed, 1 insertion(+)