Hey, thanks for reaching out. I’m familiar with the case. Because of their practice of limiting metadata retention, Open Whisper Systems itself has next to nothing to turn over, but that wouldn’t stop an attacker from sitting on a server and gathering metadata live. More on that point here.
Signal, by Open Whisper Systems, is the gold standard for secure instant messengers.
I’ve also asked Moxie about this myself, and he does not recommend it for metadata protection.