If you run above query you will get an object of type \Model\Auth_user which has an array-property "metadata" under which you can find all the properties. Simply foreach-looping over it gives you each row of the "users_metadata" table. If you only want all the metadata, you should go either that way and copy the content of $user->metadata, or you go over \Model\Auth_Metadata and query against the user's ID.
How did that solve your problem? That only shows you the SQL generated by the ORM.
Lets break your query down: - you're doing a get(), so you get an array returned - you're asking for metadata, which is a one-to-many related object, so another array
This means you need to loop twice to get all data:
foreach ($users as $user) { foreach($user->metadata as $metadata) { echo sprintf("%s: %s<br />", $metadata->key, $metadata->value); } }