>When TLS is used there is no metadata either... server x talked to server y for a while, but what, if anything, happened we know not.

Quite a bit of metadata really...

Client X: IP address

Server Y: IP address

Client X request: TLS parameters that can be analyzed through TLS fingerprinting

Server Y response: Hostnames supported by SNI

Server Y response: TLS parameters that can be analyzed thorough TLS fingerprinting