Metadata and record block access stats for indexes

Hi,

For the purpose of writing a blog post I was checking the index stats
recorded for a workload, but found them rather confusing. Following
along the code with the debugger it eventually made sense, and I could
eventually understand what's counted.  Looking around a bit, I
discovered an older discussion [1] in the mailing lists and learned that
the issue is known.  The proposal in that thread is to start counting
separate metadata and record stats depending on what type of index block
is retrieved.

I realized those would have helped me better understand the collected
index stats, so I started working on a patch to add these in the system
views. Attached is a WIP patch file with partial coverage of the B-Tree
index code. The implementation follows the existing stats collection
approach and the naming convention proposed in [1].  Let me know if what
I'm doing is feasible and if there's any concerns I could address. Next
steps would be to replace all places where I currently pass in NULL with
proper counting, as well as update tests and docs.

Looking forward to your feedback! Thanks!

Cheers,
Mircea

[1]:
https://www.postgresql.org/message-id/flat/CAH2-WzmdZqxCS1widYzjDAM%2BZ-Jz%3DejJoaWXDVw9Qy1UsK0tLA%40mail.gmail.com