Where to add Google Analytics in SharePoint 2013

You can either add the google Analytics code or any other code the old way to the Master Page or into a new APP "Web Analytics Integration"

This is a Free App that lets gives you a placeholder to add Codes to the site.

Here is the App Offical description -

"This application enables integration of third party web analytics services to your public website. Third party analytics service is integrated by injecting javascript on all site pages. The javascript to inject is determined by the third party analytics service. Examples of third party analytics services are Webtrends, Yahoo! Analytics, Google Analytics, and others."

Here is the link - Web Analytics Integration