This flake provides a NixOS module, using the existing cloud-sql-proxy in nixpkgs, to allow for easily running the proxy as a systemd service.
If you've followed the instructions here to build your NixOS system with flakes, you should be able to include this flake, and add something like the following to your configuration:
services.cloud-sql-proxy = {
enable = true;
credentials = /home/me/credentials.json;
instances = [ "myproject:myregion:myinstance?port=5432" ];
};