Add event loop utilization as a default metrics

[mcollina]

We could include https://nodejs.org/api/perf_hooks.html#performanceeventlooputilizationutilization1-utilization2 to the list of official metrics. For a few more explanations on this metrics:

https://nodesource.com/blog/event-loop-utilization-nodejs https://www.nearform.com/blog/event-loop-utilization-with-hpa

Ref: https://github.com/SkeLLLa/fastify-metrics/issues/59

[SimenB]

Yeah, sounds reasonable!