@Override
        public String toString() {
            return "MemberInfo [member.id: " + memberId
                    + ", group.instance.id: " + groupInstanceId.orElse("{}")
                    + "]";
        }
We could just use member.id to be the hashcode, since it's unique
across the group.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractPartitionAssignor.java