Get unique meta data from entities

How can I get all the unique metadata in a list of entities?

I have entities with a subtype "posts" with a metadata "area" that the user inputs in a form.

I want to get an array of all the unique metadata "area" from the entities with subtype "post"

I know the following is wrong, just an idea of what I'm trying to do.

$entities = elgg_get_entities($options);
$array_areas = $entities->area