@Override
  public String toString() {
    return "[Metadata:{" + eventTime + " " + arrivalTime + " " + scanTimeNanos + " " + scanTimeMillis + "}]";
  }
the tiemstamp when SamzSQL wuery starts processing the event
set by the SamzaSQL Scan oeprator, used by QueryTranslator to calculate
the Queuing Latency
samza-sql/src/main/java/org/apache/samza/sql/data/SamzaSqlRelMsgMetadata.java