pgsql: Add wait event for fsync of WAL segments

Add wait event for fsync of WAL segments

This has been visibly a forgotten spot in the first implementation of
wait events for I/O added by 249cf07, and what has been missing is a
fsync call for WAL segments which is a wrapper reacting on the value of
GUC wal_sync_method.

Reported-by: Konstantin Knizhnik
Author: Konstantin Knizhnik
Reviewed-by: Craig Ringer, Michael Paquier
Discussion: https://postgr.es/m/4a243897-0ad8-f471-aa40-242591f2476e@postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c55de5e5123ce58ee19a47c08425949599285041

Modified Files
--------------
doc/src/sgml/monitoring.sgml      | 4 ++++
src/backend/access/transam/xlog.c | 2 ++
src/backend/postmaster/pgstat.c   | 3 +++
src/include/pgstat.h              | 1 +
4 files changed, 10 insertions(+)