Google cannot see any message content of Signal notifications through FCM. It’s more like a “heads up” to the Signal app, telling it “hey, there are new messsges. wake up and check what they are.”. The Signal app then checks for messages and does all the decrypting and whatnot itself.

While it’s possible that the timing of FCM telling the app to check for notifications could be used to correlate activity, that’s an edge case that if you are concerned about can be easily avoided by just using the background WebSocket or a fork of Signal like Molly that allows you to use a third-party UnifiedPush provider to check for messages in the background, instead of FCM.