pg_stat_io not tracking smgrwriteback() is confusing

Hi,

I noticed that the numbers in pg_stat_io dont't quite add up to what I
expected in write heavy workloads. Particularly for checkpointer, the numbers
for "write" in log_checkpoints output are larger than what is visible in
pg_stat_io.

That partially is because log_checkpoints' "write" covers way too many things,
but there's an issue with pg_stat_io as well:

Checkpoints, and some other sources of writes, will often end up doing a lot
of smgrwriteback() calls - which pg_stat_io doesn't track. Nor do any
pre-existing forms of IO statistics.

It seems pretty clear that we should track writeback as well. I wonder if it's
worth doing so for 16? It'd give a more complete picture that way. The
counter-argument I see is that we didn't track the time for it in existing
stats either, and that nobody complained - but I suspect that's mostly because
nobody knew to look.

Greetings,

Andres Freund