Re: Fix cumulative WAL statistics double-counting from parallel workers

[shihao zhong]

Hi Lukas,

Reading v1, I think the pgstat_backend.c part makes the leader drop
the WAL of its workers from pg_stat_get_backend_wal(). A worker drops
its own backend stats entry when it exits. So on master the leader
entry is the only place that counts that WAL, and it counts it once.

I checked it with your parallel VACUUM case. On master I get

 actual_wal_bytes | pg_stat_wal_bytes | leader_backend_wal_bytes
         95926320 |         128524537 |                 97581712

With v1 I get

 actual_wal_bytes | pg_stat_wal_bytes | leader_backend_wal_bytes
         97900592 |          97581712 |                 66638887

So pg_stat_wal is right with v1, but the leader entry lost about
31MB. Is leader only intended here, to match pg_stat_get_backend_io()?
If not, maybe the pgstat_backend.c part can be left out.

Thanks,
Shihao