New statistics for tuning WAL buffer size

Hi,

It's important to provide the metrics for tuning the size of WAL 
buffers.
For now, it's lack of the statistics how often processes wait to write 
WAL because WAL buffer is full.

If those situation are often occurred, WAL buffer is too small for the 
workload.
DBAs must to tune the WAL buffer size for performance improvement.

There are related threads, but those are not merged.
https://www.postgresql.org/message-id/4FF824F3.5090407@uptime.jp
https://www.postgresql.org/message-id/flat/CAJrrPGc6APFUGYNcPe4qcNxpL8gXKYv1KST%2BvwJcFtCSCEySnA%40mail.gmail.com

What do you think?
If we can have a consensus, I will make a PoC patch.

Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION