I think this would read better as `the command requires that one of 'all', 'key' be supplied as an attribute` or better yet, use n_(): `the command requires that either 'all' or 'key' be supplied as an attribute` for 2 items, since this is a common case.

I'm also curious whether it would be more helpful to use the actual command name (`remove_metadata` or `Remove Metadata`, depending on human-readability preference), since that might help if a client is chaining commands.