That is going to be difficult, because metadata has mutliple values, so it's going to order on all of them. It might not produce the correct results, but you can try to use
@heitorfig: If you get to manage that, please let us (or at least me) know. I was trying to do a similar thing but it didn't work. Well, that's not all correct. It worked to sort by a metadata's value, but then that was the only metadata being retrieved from the database and all the other metadata attributes/keys were not assigned to the user.
So, if get this to work properly, please post an update here :)