code: @Override public ModelMetrics.MetricBuilder makeMetricBuilder(String[] domain) {
      switch(_output.getModelCategory()) {
        case Binomial:    return new ModelMetricsBinomial.MetricBuilderBinomial(domain);
        case Multinomial: return new ModelMetricsMultinomial.MetricBuilderMultinomial(domain.length,domain);
        default: throw H2O.unimpl();
      }
    }
docs: TODO: Constant response shouldn't be regression. Need to override getModelCategory()