@Override
  public void onEventComplete(MockRestRequest mockRestRequest, MockRestRequest.Event event) {
    if (eventOfInterest.equals(event) && eventOccurred.compareAndSet(false, true)) {
      eventFired.countDown();
    }
  }
Creates a listener that listens to the {@code eventOfInterest}.
@param eventOfInterest the event that this EventMonitor listens to.
ambry-rest/src/test/java/com/github/ambry/rest/AsyncRequestResponseHandlerTest.java