don't require username when certificate based authentication is used

Some servers can extract usernames from certificates. This may be as simple as extracting the common name but here is also the SPIFFE-ID which was made specifically for this purpose.

Enforcing that the user provides a username through metadata when using certificate based authentication leaves it up to the server to decide which one it should use, with some servers returning an error due to this ambiguous behaviour.

[karimra]

Agree, it makes sense to not require a username/password when running a command.