You will need to update key.json with the service account file that you get from the Firebase console.
Update the DatabaseURL inside firebase.go accordingly.
Why Firebase?
Firebase RTDB and the SDK allows you to listen to database changes out of the box. It's great for a quick project, an alternative to the pub/sub service.
Alternative
Remove all Firebase related code and replace it with your own DaaS and the rest of the project should still function well.