Include Event Loop Utilization

[mcollina]

We could include https://nodejs.org/api/perf_hooks.html#performanceeventlooputilizationutilization1-utilization2 to the list of official metrics. A few more explanation reads:

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

[SkeLLLa]

I think it's better to add this one to prom-client, so it become available not only in fastify-metrics. There are already some legacy event loop metrics, but they come from node 4 age.