Use perf_hooks monitorEventLoopDelay() instead of own monitoring

[voxpelli]

Since Node v11.10.0 Node.js has built in support for monitoring event loop delay: https://nodejs.org/api/perf_hooks.html#perf_hooks_perf_hooks_monitoreventloopdelay_options

I would suggest to use that rather than then current tracking:

prom-client/lib/metrics/eventLoopLag.js

Lines 26 to 27 in 9b4ef10

	const start = process.hrtime();
	setImmediate(reportEventloopLag, start, gauge);

[SimenB]

This is being worked on in #278

[voxpelli]

Oh, my search skills failed me 🙈 Let’s close this one then