Monitoring time of fsyncing WALs

I wonder why we are monitoring time of writing to WAL, but not time of 
fsyncing WAL segments?
Is there are principle reason for it or just because nobody added it yet?
If so, please find very small patch which adding WAIT_EVENT_WAL_FSYNC 
event type.

Our engineers in PgPro complain me that there is no information about 
time spent in syncing WALs...
Unfortunately Postgres still is not able to aggregate this statistic. 
But at least we have pg_wait_sampling extension for it:
https://github.com/postgrespro/pg_wait_sampling

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company