We generally don't consider log messages to be part of any interface contract, so I don't think you can rely on such things.
Logstash 5.x has a monitoring api that you could perhaps use to ask questions of Logstash's behavior (are things stuck, what is the slowdown, what is the event rate, etc), and maybe this will help?
Thanks, we will eventually start using monitoring API later this year. However, we would like to keep our existing monitoring system (which works well on 2.x version) before we implement new monitoring system based on API.
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