Collect eventLoopUtilization metric

nodejs supports collecting `eventLoopUtilization(ELU)` metric [out of the box](https://nodejs.org/api/perf_hooks.html#performanceeventlooputilizationutilization1-utilization2). from the [introduction article](https://nodesource.com/blog/event-loop-utilization-nodejs):
```
The simplest definition of event loop utilization (or ELU) is the ratio of
time the event loop is not idling in the event provider to the total time the event loop is running.
```
ELU can be a more helpful metric than CPU usage in % since it allows to exclude other processes from the equation. I'd suggest the APM agent collect ELU by default for the environments that support this functionality.