Restrict data checksums entries in pg_stat_io

Hi,

While reviewing the data checksums docs at [1], I found that the data checksums
launcher and workers are exposed in pg_stat_io with the same broad set of
object/context combinations as general background workers.

However, several of those entries can never accumulate I/O statistics.
For example, as far as I understand correctly, the launcher never processes
relations itself, and the workers neither operate on temporary relations nor
use the bulkread or bulkwrite contexts. So I think it would be better to
restrict the reported entries to those that the data checksums processes
can actually use.

The attached patch teaches pgstat_tracks_io_object() and pgstat_tracks_io_op()
about the actual I/O paths used by these processes. After the patch,
pg_stat_io includes only:

- data checksums launcher: relation/normal, wal/init, wal/normal
- data checksums worker: relation/normal, relation/vacuum, wal/init, wal/normal

The patch also excludes WAL reads for both processes, since they emit
WAL records but never read WAL.

Thoughts?

Regards,

[1] https://postgr.es/m/CAHGQGwFsBjQs2fv7b72hxzGV_fJMh6LAg4E83pNfDOu1jVgWCA@mail.gmail.com

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Fujii Masao