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

src/backend/postmaster/bgwriter.c

@@ -241,6 +241,7 @@ BackgroundWriterMain(void)
 
 		/* Report pending statistics to the cumulative stats system */
 		pgstat_report_bgwriter();
+		pgstat_report_wal(true);
 
 		if (FirstCallSinceLastCheckpoint())
 		{