> Say I run the server under my control, and I talk to other people on and off my server via XMPP. What is occuring is my laptop, phone & tablet are connecting back to my XMPP server, which then connects to those clients, thus not leaking my IP address, OS, etc.
Here it seems that you're defining "metadata" as your IP address (and OS?). That's kind of a non-standard definition of "metadata" in this space -- most people approach the topic more concerned about who is communicating with who.
Email is federated, and I run my own mail server, but almost every single email I send or receive has GMail at the other end of it -- so running my own server does not provide me with any meaningful metadata protection, even though it is a federated protocol. The idea that everyone in the world is going to run their own mail server (or messaging server, or whatever) has not born out in practice, even in environments that natively support federation.
I think serious metadata protection is going to require new protocols and new techniques, so we're much more likely to see major progress in centralized rather than distributed environments (in the same way that Signal Protocol is now on over two billion devices, but we're unlikely to ever see even basic large scale email end to end encryption).
If all you want to do is hide your IP address, it sounds like you should just use Tor or a VPN.
> Comparatively, as it stands now Google is getting a bunch of metadata on Signal users, such as when messages are sent and received and from which device, IP addresses, OS info, etc.
This is not true. You're referring to GCM? The only thing GCM does is wake up a device to connect to the Signal server when the app is running in the background, nothing is actually transmitted over GCM.