@Nullable byte[] getRecordKey() {
    return recordKey;
  }
Writable implementation of Kafka ConsumerRecord.
Serialized in the form:
{@code timestamp} long| {@code partition} (int) | {@code offset} (long) | {@code value.size()} (int) |
{@code value} (byte []) | {@code recordKey.size()}| {@code recordKey (byte [])}
kafka-handler/src/java/org/apache/hadoop/hive/kafka/KafkaWritable.java