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Metrics documenting re-authentications will be maintained.  They will mirror existing metrics that document authentications.  For example: failed-reauthentication-{rate,total} and successful-reauthentication-{rate,total}.  There will also be a separate successful-authentication-total metric tagged with "auth=v0" to indicate it tracks the subset of clients that successfully authenticate with a V0 SaslAuthenticateRequest.   This metric is helpful during migration (see Migration Plan for details) as it will identify if/when all clients are properly upgraded before server-side expired-connection-kill functionality is enabled: the metric will be zero across all brokers when it is appropriate to enable the feature.

An additional server-side metric expired-connections-killed-total will be created to count the number of such events.  It should remain at zero if all clients and brokers are upgraded to v2.1.0 or later.  If it is non-zero then either an older client is connecting (which would be evidenced in the tagged auth=v0 metric mentioned above) or a newer client is not re-authenticating correctly (which would indicate a bug).

A client-side metric will be created that documents the latency imposed by re-authentication.  It is unclear if this latency will be problematic, and the data collected via this metric may be useful as we consider this issue in the future.

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