New metric nodebb_eventloop_lag_seconds or similar

There is already nodejs_eventloop_lag_seconds but NodeBB is measuring this on its own and applying a smoothing factor to it. Thus to better understand 503 generated by NodeBB it would be nice to have the one measured by NodeBB. Probably it would be nice to also introduce the threshold value or probability of throttling value that can be retrieved from the toobusy-js library used by NodeBB.