[FEATURE] Add node stats thread_pool total_wait_time metric

[lukas-vlcek]

Is your feature request related to a problem?

A missing nodes stats thread_pool metric.

Some thread_pool types in the nodes stats (http://<OS_REST_end_point:9200>/_nodes/stats/?pretty) exposes "total_wait_time_in_nanos" value.

For instance index_searcher , search, search_throttled and stream_search thread_pools do.

What solution would you like?

Expose this metric as well.

What alternatives have you considered?

If this metric is not useful then do not expose it.

Do you have any additional context?
n/a

[andrross]

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