From the proposed doc updates by Darrel Herbst:
Hugo ships with prebuilt internal templates for Google Analytics tracking, including both synchronous and asynchronous tracking codes.
Configuring Google Analytics
Provide your tracking id in your configuration file, e.g. config.yaml.
googleAnalytics = "UA-123-45"
Example
Include the internal template in your templates like so:
{{ template "_internal/google_analytics.html" . }}
For async include the async template:
{{ template "_internal/google_analytics_async.html" . }}