performance : pass unique(values) to metricSpecificType

[Mathis Derenne]

unique() is called multiple time when creating a model.column.
This happen because it needed at both level : GenericMetric, SpecificMetric.

Solution : make SetGenericMetric() return Unique() and pass it to SetStringMetric and SetNumericMetric.

[Mathis Derenne]

StringMetric do not make use of Unique() but build LenCounter and LenMap.
NumericMetric and BoolMetric do not make use of Unique().

Closing issue