@Override
  public String toString() {
    return "ConsumerGroupMeta{"
        + "consumerGroupId='"
        + consumerGroupId
        + "', creationTime="
        + getCreationTime()
        + ", topicNameToSubscribedConsumerIdSet="
        + topicNameToSubscribedConsumerIdSet
        + ", consumerIdToConsumerMeta="
        + consumerIdToConsumerMeta
        + ", topicNameToSubscriptionCreationTime="
        + topicNameToSubscriptionCreationTime
        + "}";
  }
@return topics subscribed by no consumers in this group after this removal.
@param consumerId if null, remove subscriptions of topics for all consumers
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/subscription/meta/consumer/ConsumerGroupMeta.java