@Override
  public String toString() {
    return MoreObjects.toStringHelper(this).toString();
  }
Applies the {@link StartpointVisitor}'s visit methods to the {@link Startpoint}
and returns the result of that operation.
@param input the metadata associated with the startpoint.
@param startpointVisitor the visitor of the startpoint.
@return the result of applying the visitor on startpoint.
samza-api/src/main/java/org/apache/samza/startpoint/Startpoint.java