Why does [auto-]vacuum delay not report a wait event?

Hi,

I was looking at [1], wanting to suggest a query to monitor what
autovacuum is mostly waiting on. Partially to figure out whether it's
mostly autovacuum cost limiting.

But uh, unfortunately the vacuum delay code just sleeps without setting
a wait event:

void
vacuum_delay_point(void)
{
...
/* Nap if appropriate */
if (msec > 0)
{
if (msec > VacuumCostDelay * 4)
msec = VacuumCostDelay * 4;

pg_usleep((long) (msec * 1000));

Seems like it should instead use a new wait event in the PG_WAIT_TIMEOUT
class?

Given how frequently we run into trouble with [auto]vacuum throttling
being a problem, and there not being any way to monitor that currently,
that seems like it'd be a significant improvement, given the effort?

It'd probably also be helpful to report the total time [auto]vacuum
spent being delayed for vacuum verbose/autovacuum logging, but imo
that'd be a parallel feature to a wait event, not a replacement.

Greetings,

Andres Freund

[1] https://postgr.es/m/CAE39h22zPLrkH17GrkDgAYL3kbjvySYD1io%2BrtnAUFnaJJVS4g%40mail.gmail.com