@Override
  public boolean isOpen() {
    boolean isOpen = channelOpen.get();
    onEventComplete(Event.IsOpen);
    return isOpen;
  }
Called when an event (function call) finishes successfully in MockRestResponseChannel. Does *not* trigger if the
event (function) fails.
@param mockRestResponseChannel the {@link MockRestResponseChannel} where the event occurred.
@param event the {@link Event} that occurred.
ambry-test-utils/src/main/java/com/github/ambry/rest/MockRestResponseChannel.java