Add event loop idle and active time

[xamgore]

The eventLoopUtilization() method returns an object that contains the cumulative duration of time the event loop has been both idle and active as a high-resolution milliseconds timer.

With the new release, there are two extra metrics: nodejs/node#34938

[alex3d]

I've added event loop idle metric in #474
I'm not sure whether the active metric is really needed (actually it is derived from idle in node source code https://github.com/nodejs/node/blob/master/lib/internal/perf/event_loop_utilization.js).
Event loop utilization could be calculated as 1 - rate(nodejs_eventloop_idle_seconds_total).

[jdmarshall]

Would active time be assumed to include time while for instance a *Sync IO function is called? If so then 1-idle would not be correct.

[jdmarshall]

#785 covers enough of this that I think this can be closed.