public CoordinatorResult<Void, CoordinatorRecord> classicGroupJoin(
        RequestContext context,
        JoinGroupRequestData request,
        CompletableFuture<JoinGroupResponseData> responseFuture
    ) {
        return groupMetadataManager.classicGroupJoin(
            context,
            request,
            responseFuture
        );
    }
Handles a JoinGroup request.

@param context The request context.
@param request The actual JoinGroup request.

@return A Result containing the JoinGroup response and
        a list of records to update the state machine.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupCoordinatorShard.java