Move wal_buffers_full from PgStat_PendingWalStats to WalUsage

wal_buffers_full has been introduced in pg_stat_wal in 8d9a935965, as
some information providing metrics for the tuning of the GUC
wal_buffers.  WalUsage has been introduced before that in df3b181499.

Moving this field is proving to be beneficial for several reasons:
- This information can now be made available in more layers, providing
more granularity than just pg_stat_wal, on a per-query basis: EXPLAIN,
pgss and VACUUM/ANALYZE logs.
- A patch is under discussion to provide statistics for WAL at backend
level, and this move simplifies a bit the handling of pending
statistics.  The remaining data in PgStat_PendingWalStats now relates to
write/sync counters and times, with equivalents present in pg_stat_io,
that backend statistics are able to already track.  So this should cut
all the dependencies between PgStat_PendingWalStats and WAL stats at
backend level.

As of this change, wal_buffers_full only shows in pg_stat_wal.

Author: Bertrand Drouvot
Reviewed-by: Ilia Evdokimov
Discussion: https://postgr.es/m/Z6SOha5YFFgvpwQY@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/executor/instrument.c

@@ -280,6 +280,7 @@ WalUsageAdd(WalUsage *dst, WalUsage *add)
 	dst->wal_bytes += add->wal_bytes;
 	dst->wal_records += add->wal_records;
 	dst->wal_fpi += add->wal_fpi;
+	dst->wal_buffers_full += add->wal_buffers_full;
 }
 
 void
@@ -288,4 +289,5 @@ WalUsageAccumDiff(WalUsage *dst, const WalUsage *add, const WalUsage *sub)
 	dst->wal_bytes += add->wal_bytes - sub->wal_bytes;
 	dst->wal_records += add->wal_records - sub->wal_records;
 	dst->wal_fpi += add->wal_fpi - sub->wal_fpi;
+	dst->wal_buffers_full += add->wal_buffers_full - sub->wal_buffers_full;
 }