With this PR, we also need to update places that parse user:password and do not expect : to be a part of the user name. For example, in https://github.com/coreos/etcd/blob/master/etcdctl/ctlv3/command/global.go#L351 :

	splitted := strings.SplitN(userFlag, ":", 2)
	if len(splitted) < 2 {
		cfg.username = userFlag
		cfg.password, err = speakeasy.Ask("Password: ")
		if err != nil {
			ExitWithError(ExitError, err)
		}
	} else {
		cfg.username = splitted[0]
		cfg.password = splitted[1]
	}

Also, the documentation around using a colon strictly as a password separator needs to be updated, as well as adding in new notes about the separate --password flag that mention colons in the username will no longer be treated specially when the password flag is given.

The example I posted above should not be taken as an exhaustive list of places where colons aren't expected to be a part of the username, but just the first example I found. My main point is that allowing a username with a colon in it is just the first step in adding full support for colons in usernames. I'd rather have a single PR for full support rather than merging in code that could leave the system in a weird state: the user exists, but can't login.