A callback invoked when proactive rate-limit pacing delays the next request
after a successful attempt, with the pacing wait in delayMs. Fires after
a completed wait, before the request dispatches, and for a wait aborted
partway through: an aborted wait reports the elapsed portion in delayMs
with aborted: true, so observers can distinguish a cancelled pacing wait
from no pacing at all without watching onError.
A callback invoked when proactive rate-limit pacing delays the next request after a successful attempt, with the pacing wait in
delayMs. Fires after a completed wait, before the request dispatches, and for a wait aborted partway through: an aborted wait reports the elapsed portion indelayMswithaborted: true, so observers can distinguish a cancelled pacing wait from no pacing at all without watchingonError.