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Proposed tag set : security protocol, listener name, network processor id

The listener tag could be omitted if it is the same as the protocol one :

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Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
  
  Users currently using reporters that do not support metric tags and flatten the tags map into the metrics name, will find that the name has changed
• The socket-server-metrics.* are not exposed as Yammer metrics which are still the most commonly consumed server-side metrics, so the impact of this change is mitigated (thanks Ismael Juma Juma)

Rejected Alternatives

The listener tag could be omitted if it is the same as the protocol one :

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rejected as Listener names and security protocols are different concepts (even if the default value is the same for compatibility reasons)
and it's a bit odd to expect generic tools that process Kafka metrics to have to fallback to the security protocol if listener name is missing.