Hi hackers,

The write(2) calls that flush server log output aren't covered by wait
events. When a backend logs something, the writes go out in:

  - write_pipe_chunks(): write(2) to the syslogger pipe
  - write_console(): write(2) to stderr (WriteConsoleW() on Windows)

If one of those blocks -- syslogger pipe full, slow console, slow log
device -- pg_stat_activity just shows wait_event = NULL until it
returns. Since NULL usually reads as "on CPU", a backend stuck writing
logs looks like it's doing work, so logging-related stalls are easy to
miss.

Attached is a short series that adds two WaitEventIO events and reports
them around those writes:

  IO / SysloggerWrite - write(2) to the syslogger pipe
  IO / StderrWrite - write(2) to stderr, and WriteConsoleW()

0001 adds the events and covers the write(2) paths. 0002 does the
Windows WriteConsoleW() path, split out since it's platform-specific.

It only wraps the leaf write call and uses the existing
pgstat_report_wait_start()/end() helpers, so it stays allocation-free
and safe to call from inside the error-reporting path.

I did a quick before/after to make sure the events show up: 8 backends
each emitting large RAISE LOG lines, sampling wait_event from
pg_stat_activity every 50 ms for 20 s.

  - logging_collector = on (syslogger pipe):
    master:  NULL                100.0%  (2184/2184)
    patched: IO/SysloggerWrite     99.1%  (2204/2224), NULL 0.9%

  - logging_collector = off (stderr):
    master:  NULL                100.0%  (2144/2144)
    patched: IO/StderrWrite        90.7%  (1952/2152), NULL 9.3%

On master that wait time is just invisible; with the patch it lands on
the new events. I can send the scripts and raw samples if anyone wants
to reproduce it.