Allows users to configure the GroupInstanceID

[huszkacs]

GroupInstanceID can be configured by setting ReaderConfig.GroupInstanceID which will be passed to the joinGroup and heartbeat requests. If the GroupInstanceID has a value the kafka server will set it as a static member.

[rhansen2]

Hi @huszkacs,

Thanks for submitting this PR!. Just wanted to clarify if this PR is meant to fully support the behavior described in https://cwiki.apache.org/confluence/display/KAFKA/KIP-345:+Introduce+static+membership+protocol+to+reduce+consumer+rebalances#KIP345:Introducestaticmembershipprotocoltoreduceconsumerrebalances-ClientSideChanges or if you're just looking to support passing the GroupInstanceID?

[huszkacs]

@rhansen2 it is meant to fully support the behaviour. Thanks to the new structure, there is no need for any other change. Previously I had more complicated PR.

(sorry for the late answer)

[rhansen2]

It looks like https://cwiki.apache.org/confluence/display/KAFKA/KIP-345:+Introduce+static+membership+protocol+to+reduce+consumer+rebalances describes some changes to the client behavior, specifically For the effectiveness of the KIP, consumer with group.instance.id set will not send leave group request when they go offline and some changes to the range balancer. Are we already compliant with those behaviors?

[huszkacs]

that is a good question, I take a look

[rhansen2]

Closed due to inactivity as well as some of the changes this depended on being reverted.