[racket-dev] Measuring # of waiters on a port?

I'm trying to isolate a "503" error coming from WeScheme occasionally,
and I think I've isolated the problem down to the number of allowed
waiters on a port.

I see that web-server has the default set to 40.

Is there a good way to tell whether or not we do reach this limit at
runtime, or is that not observable from Racket?  I suspect I'm hitting
this limit, but if there's a good way to surface this, I'd be more
comforted.