... <function>XLogWrite</function>, which is invoked during an <function>XLogFlush</function> request (see ...). This is also incremented by the WAL receiver during replication.
("which normally called" should be "which is normally called" or "which normally is called" if you want to keep true to the original)
You missed the adding the space before an opening parenthesis here and elsewhere (probably copy-paste)
is ether -> is either
"This parameter is off by default as it will repeatedly query the operating system..."
", because" -> "as"
wal_write_time and the sync items also need the note: "This is also incremented by the WAL receiver during replication."
"The number of times it happened..." -> " (the tally of this event is reported in wal_buffers_full in....) This is undesirable because ..."
I notice that the patch for WAL receiver doesn't require explicitly computing the sync statistics but does require computing the write statistics. This is because of the presence of issue_xlog_fsync but absence of an equivalent pg_xlog_pwrite. Additionally, I observe that the XLogWrite code path calls pgstat_report_wait_*() while the WAL receiver path does not. It seems technically straight-forward to refactor here to avoid the almost-duplicated logic in the two places, though I suspect there may be a trade-off for not adding another function call to the stack given the importance of WAL processing (though that seems marginalized compared to the cost of actually writing the WAL). Or, as Fujii noted, go the other way and don't have any shared code between the two but instead implement the WAL receiver one to use pg_stat_wal_receiver instead. In either case, this half-and-half implementation seems undesirable.
