Elasticsearch Version

ALL

Installed Plugins

No response

Java Version

bundled

OS Version

Darwin

Problem Description

The GET _ilm/policy API could return many records (for clusters that have
many ILM policies).
The API runs on the transport_worker thread so this can cause master instability
issues ( https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/ilm/action/TransportGetLifecycleAction.java#L58 ).

This proposes moving the API on the MANAGEMENT thread pool and making
the API cancellable.

Steps to Reproduce

Have thousands of ILM policies.
Repeatedly issue GET _ilm/policy

Logs (if relevant)

No response