def measure(config)
      reduce(MeasurementSet.new) do |set, document|
        set.merge(Document.measure(document, config))
      end
    end
def measure(config)
      reduce(MeasurementSet.new) do |set, document|
        set.merge(Document.measure(document, config))
      end
    end
Measure documents using given config

 @return [Yardstick::MeasurementSet]
   a collection of measurements

 @api private