Attach a long event loop delay span to an APM transaction

[mshustov]

Since v7.14 PR, Kibana reports a warning if the mean value of event loop delay exceeds 350ms. It helps users spot a performance problem but not investigate it since the runtime context is absent.

To overcome the problem, we can borrow a few ideas from [this article] (https://www.ashbyhq.com/blog/engineering/detecting-event-loop-blockers).
TLDR: Server can capture the runtime context of the expensive tasks by implementing a custom async hook tracking the duration of a task and attaching it to an APM transaction. It allows Cloud customers quickly identify what APM transaction triggers CPU-bound tasks on the Kibana server.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[lukeelmers]

Looks like somehow this issue got created twice? #128647

I'll go ahead and close this one as a duplicate