I'm really late with the reply and it doesn't really matter anyway, but how Signal is doing it is not what I would like to see. It would be less of an issue if you can backup and restore your Signal chats, but as of right now, it's impossible (at least on Apple devices). I use Signal because lots of my friends prefer it, but I don't like that losing my phone would mean losing my complete chat history of all my chats. Maybe I'm not their target audience, but I think Signal is already mainstream enough that chat backups and restores should be a thing.

What I was expecting from Telegram (although it doesn't look like they plan to do this) is synced, E2E encrypted cloud chats. So any new device I add has access to all the previous message history, and is independent of all other devices.