public void onSubscriptionUpdated() {
        if (state == MemberState.UNSUBSCRIBED) {
            transitionToJoining();
        }
    }
Join the group with the updated subscription, if the member is not part of it yet. If the
member is already part of the group, this will only ensure that the updated subscription
is included in the next heartbeat request.
<p/>
Note that list of topics of the subscription is taken from the shared subscription state.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/ShareMembershipManager.java