@Override
  public String toString() {
    return "ConsumerGroupMetaKeeper{"
        + "consumerGroupIdToConsumerGroupMetaMap="
        + consumerGroupIdToConsumerGroupMetaMap
        + '}';
  }
Get the consumers subscribing the given topic in the given consumer group.

@param topic The topic name.
@return The set of consumer IDs subscribing the given topic in this group. If the group does
    not exist or no consumer is subscribing the topic, return an empty set.
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/subscription/meta/consumer/ConsumerGroupMetaKeeper.java