refs #4706 added possibility to register a closure or a callable method to process processed metrics as late as possible and to make sure it always returns the correct value whenever it is called. Otherwise the value does not get updated in case for instance the nb_visits column changes (done in SumRow). Whenever there is a column that is not numeric and not the label column it should use a callback method instead of calculating the value directly to make sure the correct value is returend when using flat or truncate or ...