Hi @dbME—Fly.io’s reverse proxy sets all of the X-Forwarded headers listed here by default, so you shouldn’t have to do anything on the Fly.io side to have them sent to your app. (The debug.fly.dev app echoes back all of the headers that it receives, so you can also see which headers are set there.)
I’m not familiar with Spring Boot or Vaadin, though, so unfortunately I can’t provide any advice about configuring your app to use these headers.
(One other thing to be aware of: the X-Forwarded headers can be overridden by clients. If this is undesirable or insecure for your app, then the Fly-Forwarded-Proto header has the same function as X-Forwarded-Proto but is not overridable.)