[BUG] Logging flood on search rejections

[jainankitk]

Describe the bug
Search rejection for every shard is being logged even when the debug logging is not enabled. Logging an exception is very expensive as the resulting message can be big wreaking havoc on memory, CPU (due to GC) and disk throughput. Regression commit came from ES 7.10 here.

Search rejection is a mechanism for ensuring resiliency and preventing the cluster from getting overwhelmed. Because of this issue, the cluster degrades further once we see search thread pool rejections

To Reproduce
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4. See error

Expected behavior
The logging should only happen when the debug log is enabled

Plugins
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Host/Environment (please complete the following information):

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Additional context
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[jainankitk]

Closing this issue as PR was merged!