code: public static void setPartitionAvgRecordMillis(State state, KafkaPartition partition, double millis) {
      state.setProp(
          getPartitionPropName(partition.getTopicName(), partition.getId()) + "." + KafkaSource.AVG_RECORD_MILLIS,
          millis);
    }
docs: Set the average time in milliseconds to pull a record of a partition, which will be stored in property
  "[topicname].[partitionid].avg.record.millis".