bgwriter doesn't flush WAL stats

Hi,

I was trying to add WAL stats to pg_stat_io. While doing that I was
comparing pg_stat_wal and pg_stat_io's WAL stats and there was some
inequality between the total number of WALs. I found that the difference
comes from bgwriter's WALs. bgwriter generates WAL but it doesn't flush
them because the pgstat_report_wal() function isn't called in bgwriter. I
attached a small patch for calling the pgstat_report_wal() function in
bgwriter.

bgwriter generates WAL by calling functions in this order:
bgwriter.c -> BackgroundWriterMain() -> BgBufferSync() -> SyncOneBuffer()
-> FlushBuffer() -> XLogFlush() -> XLogWrite()

I used a query like BEGIN; followed by lots of(3000 in my case) INSERT,
DELETE, or UPDATE, followed by a COMMIT while testing.

Example output before patch applied:

┌─────────────┬─────────────────┐
│  view_name  │ total_wal_write │
├─────────────┼─────────────────┤
│ pg_stat_wal │           10318 │
│ pg_stat_io  │           10321 │
└─────────────┴─────────────────┘

┌─────────────────────┬────────┬────────┐
│    backend_type     │ object │ writes │
├─────────────────────┼────────┼────────┤
│ autovacuum launcher │ wal    │      0 │
│ autovacuum worker   │ wal    │    691 │
│ client backend      │ wal    │   8170 │
│ background worker   │ wal    │      0 │
│ background writer   │ wal    │      3 │
│ checkpointer        │ wal    │      1 │
│ standalone backend  │ wal    │    737 │
│ startup             │ wal    │      0 │
│ walsender           │ wal    │      0 │
│ walwriter           │ wal    │    719 │
└─────────────────────┴────────┴────────┘

After the patch has been applied, there are no differences between
pg_stat_wal and pg_stat_io.

I appreciate any comment/feedback on this patch.

Regards,
Nazir Bilal Yavuz
Microsoft