List custom thread pools in _cat/thread_pool

[avlukanin]

Describe the feature:

It is easy to create a custom thread pool in elasticsearch.yml, e.g.:

threadpool.asdf.type: fixed
threadpool.asdf.size: 10

But this thread pool is not listed, when you use _cat/thread_pool?v=asdf command, because only SUPPORTED_NAMES can be listed in org.elasticsearch.rest.action.cat.RestThreadPoolAction#getTableWithHeader

[clintongormley]

We've been talking about adding a class which is responsible for creating threads. Any other class that needs a thread pool should register its requirements with this class, and they should be available in the stats.

Related to #17915 and #12666

[avlukanin]

The stats is already collected for custom thread pools, but not shown. I had to patch RestThreadPoolAction to add my custom thread pool name to SUPPORTED_NAMES and SUPPORTED_ALIASES to show it.

Can you confirm, that the stats for custom thread pools will not only be collected, but shown as well, when the new paradigm is implemented?

[elasticmachine]

Pinging @elastic/es-core-infra

[martijnvg]

This was fixed in #19721.