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: SkeLLLa/fastify-metrics#59