I think you misread woah's comment. Signal has access to the metadata but chooses not to store it in order to be able to remain unresponsive to queries like this.
So the real threat here is that the FBI wants to come in with a search warrant, install one of their famous splitters (a Windows machine btw) and routes all metadata traffic to the NSA.
To legally get contact data in real-time for one criminal under investigation. Essentially the Lavabit case.
Or is that what they called PRISM? Legal route splitting at the endpoint, i.e. legally declared as such by the secret FISA court, because there's oversight...
