Hi @Dmitry1,
it means that Elasticsearch prevented some requests from executing to avoid an out of memory error, see documentation on circuit breaker. From the snippet, it looks like this is when speaking to your monitoring cluster.
If you use G1 GC, using the JVM settings from this PR might help: https://github.com/elastic/elasticsearch/pull/46169.
If your heap is running high, you should consider either scaling out or reducing the amount of shards (though a more thorough analysis is necessary to really conclude on the root cause). If your production and monitoring cluster is the same, it could make sense to split into two clusters.