Today, Elasticsearch has a separate merge thread pool checking once
per second (by default) if any merges are necessary, but this is no
longer necessary since we can and do now tell Lucene's
ConcurrentMergeScheduler never to "hard pause" threads when merges
fall behind, since we do our own index throttling.
This change goes back to letting Lucene launch merges as needed, and
removes these two expert settings:
index.merge.force_async_merge
index.merge.async_interval
Now merges kick off immediately instead of waiting up to 1 second
before running.
Closes#8643
By default active, rejected and queue thread statistics are included for the index, bulk and search thread pool.
Other thread statistics of other thread pools can be included via the `h` query string parameter.
Closes#4907