code: func (f *Factory) Histogram(options metrics.HistogramOptions) metrics.Histogram {
  	histogram := &histogram{
  		histograms: make([]metrics.Histogram, len(f.factories)),
  	}
  	for i, factory := range f.factories {
  		histogram.histograms[i] = factory.Histogram(options)
  	}
  	return histogram
  }
docs: // Histogram implements metrics.Factory interface