Task Manager health

Analyze the output of the Kibana Task Manager health API, from GET kbn:/api/task_manager/_health in Dev Tools. Multiple health reports (for instance, one from each Kibana instance) can be provided together. A "Latest Monitored Stats" Kibana log message also works.

The health report is only complete from Kibana instances with the background_tasks node role. On Elastic Cloud Hosted with Kibana larger than 8GB, and always on serverless, the API request goes to a UI instance, so get the report from the "Latest Monitored Stats" messages in the Kibana logs instead.