Proxy is a rate limiting Google Cloud Function that knows how to proxy authenticated traffic to Google Cloud Run backend services.
Proxy interacts with a Redis instance which, on Google Cloud Platform, is a private VPC service. At the time of writing, Cloud Run services have no ability to connect to private VPC services, hence the need for a Cloud Function.
Proxy is pretty tightly coupled with COVID Trace services so it's really only useful in the context of deploying a COVID Trace stack. The code is pretty simple to follow, check out proxy.go to better understand how it works and deploy.sh to see how it is deployed.