Inspect and report on event loop activity from other thread

[mrocklin]

The health of the event loop is important for the smooth functioning of the workers and scheduler. Is it possible to periodically inspect what the event loop is doing and report that with some sort of diagnostic? This might help advanced users to diagnose tricky performance problems.

cc @pitrou

If we get a good data feed on this then it might be reasonable to turn it into a diagnostic plot.