Ideally, the government login/auth would be an opt-in for sites where anonymity isn't important. Facebook, for example, already has a real-name policy but still has fake accounts. Moderation alone isn't sufficient; it's hard to keep up with the number of bad actors. Limiting signups to verified humans, and possibly validating their nationality, can help with that IRT to bots and foreign agents.

It's important that the mechanism be opt-in, though, and yeah, metadata would be a problem. But realistically it's just a matter of degree... they already have access to all of that metadata today with just a subpoena or national security letter. Centralizing the login would make it easier for them to collect it, but also make it easier to audit via government mechanisms (FOIAs, etc.) compared to the opacity of private companies (which are under zero obligation to reveal how things are stored).