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