Signal servers know all message metadata (timestamp, hash of participants ID, message size), the only thing they don't know is message content. If that model is ported to nostr, relays would know all metadata and if they don't restrict who can query it, any user can know all metadata.
Discussion
You can't directly port it, but does it make sense to come up with a totally new protocol, instead of extending signal to these capabilities?
It already exists and it's already working, it's not a science experiment.
Signal is extended to nostr?
I meant that a new more private working protocol exists (simpleX), not that it was ported to nostr.
Do you have a link to the protocol description?