Hot threads should filter out "uninteresting" thread stacks by default

[mikemccand]

When I stare at a hot threads output, e.g. #8905, many of threads are clearly unimportant, because e.g. they are in parked in a lock under ThreadPoolExecutor.getTask, or inside "select" in Netty.

I think we should filter these out by default?

[dadoonet]

+1. I am also wondering if someone looks at thread stack when it's less than 1ms of 500ms...

[bleskes]

+1 to an optional, turned on by default, filtering of known stack traces which are useless. Potentially make this also dependent on thread names? i.e., search , index etc.

[nik9000]

For completeness sake #5952 is one of the primary reasons I've seen these threads show up. hot_threads in general can really overemphasize long running tasks like merge over shorter queries that just happen to jump to another thread. Its the price for hot_threads being low impact on a running cluster though.