I don't think exposing the information about whether the connection is metered or not is acceptable from the privacy standpoint. Based on the IP address of a user & this metered status, a website may even be able to tell what kind of carrier plan a given user is in.
Thanks for the reply, Ryosuke! Just to clarify, the `metered` attribute would be a manual user setting, not a browser heuristic. This means you could easily mark your all-data included WiFi at home as metered if you wanted to, or, on the opposite end, mark your roaming data plan you purchased for a ton of money at the airport as unmetered. None of this happens without the user voluntarily revealing the information.