Add shared buffer hits to pg_stat_io

Hi,

As suggested in [1], the attached patch adds shared buffer hits to
pg_stat_io.

I remember at some point having this in the view and then removing it
but I can't quite remember what the issue was -- nor do I see a
rationale mentioned in the thread [2].

It might have had something to do with the interaction between the
now-removed "rejected" buffers column.

I am looking for input as to the order of this column in the view. I
think it should go after op_bytes since it is not relevant for
non-block-oriented IO. However, I'm not sure what the order of hits,
evictions, and reuses should be (all have to do with buffers).

While adding this, I noticed that I had made all of the IOOP columns
int8 in the view, and I was wondering if this is sufficient for hits (I
imagine you could end up with quite a lot of those).

- Melanie

[1] https://www.postgresql.org/message-id/20230209050319.chyyup4vtq4jzobq%40awork3.anarazel.de
[2] https://www.postgresql.org/message-id/flat/20230209050319.chyyup4vtq4jzobq%40awork3.anarazel.de#63ff7a97b7a5bb7b86c1a250065be7f9