Care to share what you mean by PGP leaks a lot of metadata? You might be right, I'm just not aware of such details.
With signal this is much harder, for instance google/apple do not know who you are chatting to. They also don't handle the encrypted message transport/delivery.
OWS received a Grand jury subpoena and was only able to produce "the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user's connectivity to the Signal service.".
Certainly a NSL might compel OWS to add additional logging (and not talk about it). With that they could tell who messaged who, when the message was sent, and how big the message was.
NSLs cannot be used for that. They're a legal tool that can be used to extract certain types of information (such as subscriber information and maybe a little bit of transactional information) that a service provider already has stored on their servers [0]. However, they cannot be used to force a service provider to write and deploy code.
[0] NSLs are not magic - https://www.youtube.com/watch?v=YN_qVqgRlx4&t=20m16s
Anything that does TLS to connect to a single server can do that. Heck, if you do email to a single email server using secure IMAP and secure SMTP then PGP no longer leaks metadata.
