Handle ELU in worker threads

[dnlup]

Node API as a reference:

• worker: add eventLoopUtilization() nodejs/node#35664

This also opens up a question: should this module be worker_thread aware? Meaning, if it's running in a worker thread, should it forward metrics to the main thread?

A possible API could be sampler.addWorker(). This would have to take care of setting up worker-main thread communication.

Also, workers could be passed in the Sampler constructor.