App Engine service that creates a POST API endpoint. The endpoint digests a JSON payload instructing what type of cookies to build into the Set-Cookie header.
Safari's Intelligent Tracking Prevention caps the expiration of JavaScript cookies to max. 7 days. This is an inconvenience to first-party web analytics, which relies on persistent browser cookies for stitching together hits from a single web browser.
The point of the web service is to offer an API that sets cookies in the Set-Cookie HTTP response header. This alone wouldn't be very useful, as these headers would be ineffective due to being sent cross-origin.
However, you can set up a custom domain mapping to the Google App Engine endpoint, which effectively lets you service this API in your own domain namespace. Thus any cookies sent with the Set-Cookie header are treated as first-party cookies by the browser.
Guide: https://www.simoahava.com/google-cloud/create-cookie-rewrite-web-service-google-cloud/
Video: https://youtu.be/cmRrSeR51tc