pg_stat_wal: tracking the compression effect

Hi hackers,

We can specify compression method (for example, lz4, zstd), but it is 
hard to know the effect of compression depending on the method. There is 
already a way to know the compression effect using pg_waldump. However, 
having these statistics in the view makes it more accessible. I am 
proposing to add statistics, which keeps track of compression effect in 
pg_stat_ wal view.

The design I am thinking is below:

compression_saved | compression_times
------------------+-------------------
             38741 |                6


Accumulating the values, which indicates how much space is saved by each 
compression (size before compression - size after compression), and keep 
track of how many times compression has happened. So that one can know 
how much space is saved on average.

What do you think?

Regards,

-- 
Ken Kato
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION