code: def google_analytics(web_property_id, *args)
        require "garb"
        options = Hash === args.last ? args.pop : {}
        metric = args.shift || :pageviews
        @ga_resource = Vanity::Metric::GoogleAnalytics::Resource.new(web_property_id, metric)
        @ga_mapper = options[:mapper] ||= lambda { |entry| entry.send(@ga_resource.metrics.elements.first).to_i }
        extend GoogleAnalytics
      rescue LoadError
        fail LoadError, "Google Analytics metrics require Garb, please gem install garb first"
      end
docs: Use Google Analytics metric. Note: you must +require "garb"+ before
   vanity.
  
   @example Page views
     metric "Page views" do
       google_analytics "UA-1828623-6"
     end
   @example Visits
     metric "Visits" do
       google_analytics "UA-1828623-6", :visits
     end
  
   @since 1.3.0
   @see Vanity::Metric::GoogleAnalytics