code: void open(int groupSize) {
      try {
        close();
        ConsumerConfig config = consumerConfig;
        if (groupSize != config.getGroupSize()) {
          config = new ConsumerConfig(consumerConfig.getGroupId(),
                                      consumerConfig.getInstanceId(),
                                      groupSize,
                                      consumerConfig.getDequeueStrategy(),
                                      consumerConfig.getHashKey());
        }
        if (queueName.isQueue()) {
          QueueConsumer queueConsumer = dataFabricFacade.createConsumer(queueName, config, numGroups);
          consumerConfig = queueConsumer.getConfig();
          consumer = queueConsumer;
        }
      } catch (Exception e) {
        throw Throwables.propagate(e);
      }
    }
docs: Updates number of instances for the consumer group that this instance belongs to. It'll close existing
  consumer and create a new one with the new group size.
  
  @param groupSize New group size.