This code is for serving an AMP GTM container from your own domain, so that the GA Client ID can be shared between your AMP pages and the rest of your site (enabling them to be tracked together in the same GA property).
Once the endpoint is deployed, you'll need to replace
//www.googletagmanager.com/amp.json in the
<amp-analytics> embed with the path to your endpoint (leaving the query parameters in place) within your AMP template's source code:
You can specifiy the cookie domain by appending
&cookieDomain=YOURDOMAIN.COM to the endpoint URL.