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In this process, there is no visibility from broker side to see what portion of the clients have been upgraded. Therefore changing the message format version becomes a difficult decision. 

Proposed Changes

Previous metric: kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...}

New metric: kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...},version=INTEGER

We want to amend RequestsPerSec metric to have "version" tag so that we have the insight of versions that clients are using. The value is an integer that represent the version for the specific request type. This can be done in RequestChannel where it already parses an keeps the client API version in memory and updates the RequestsPerSec metric. With this change, an additional hash lookup is needed when updating the metric to locate the metric corresponding to a specific version.

The metrics for all versions will be cleaned up at broker shutdown.

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