code: public static void setPartitionAvgRecordSize(State state, KafkaPartition partition, long size) {
      state.setProp(getPartitionPropName(partition.getTopicName(), partition.getId()) + "." + KafkaSource.AVG_RECORD_SIZE,
          size);
    }
docs: Set the average record size of a partition, which will be stored in property
  "[topicname].[partitionid].avg.record.size".