How to setup port forwarding on Google Cloud
Solution 1:
What you're referring to is actually called Port Address Translation (PAT), and is not directly supported by Google Cloud Platform.
Instead, you might be able to reconfigure the software in question to listen on the desired port (6984).
EDIT: Let's Encrypt is sunsetting and deprecating TLS-SNI challenges, so you'll need to use HTTP-01 (port 80) or DNS-01 (DNS record) for Let's Encrypt instead -- so, if you use one of those, you can have CouchDB run on any port you want other than port 80.