KAFKA-18836 Make ConsumerGroupMetadata an interface

[pszymczyk]

Implementation for KAFKA-18836 improvement.
KIP-1136

[frankvicky]

Hi @pszymczyk
Thanks for the patch
According to the ticket, this change should have a KIP first since this will introduce a change to the public API.

Please refer to the following link for further information:
https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals

[pszymczyk]

Hi @frankvicky
Sure I will provide a KIP this week, just need to read all the manuals and follow procedures as it is my first contribution into Kafka project.

[mjsax]

We cannot do it this way. For Kafka 4.1, we need to keep the class as-is, to maintain backward compatibility. We should only deprecate both constructors, and update the JavaDocs saying, that this class will become an interface in the future, ie, Apache Kafka 5.0.

The KIP should lay out this two step approach, and if the KIP gets accepted, we should file a follow up ticket for AK 5.0 release, that would do the actual code change from class to interface.

[pszymczyk]

Sure, makes sense, I will provide a KIP and update the PR.

[pszymczyk]

@mjsax KIP https://cwiki.apache.org/confluence/display/KAFKA/KIP-1136%3A+Make+ConsumerGroupMetadata+an+interface