@Override
  public String toString() {
    return "RelMessage: {" + samzaSqlRelRecord + " " + samzaSqlRelMsgMetadata + "}";
  }
Creates the SamzaSqlRelMessage from {@link SamzaSqlRelRecord}.
@param samzaSqlRelRecord represents the rel record.
@param metadata the message/event's metadata
samza-sql/src/main/java/org/apache/samza/sql/data/SamzaSqlRelMessage.java