As the meta tag contains tree values we would need tree too. Sonatas `PageService` adds the option for that by using the following method: `$pageService->addMetas($type, $key, $value)`. As i have seen in my failing tests, those types, keys and values arn't realy tested/checked.
The existing properties of this bundle are using the same interface (except for canonical links and the title). So a description is set by using `...->addMetas('name', 'description', 'value for description');` We could add them in the same mechanism, but i i do not want to as those properties a a little bit special and got its own logic from combining content's values with default one.

I would suggest the tricky solution too: `property_type_key = value`. But there would be a second solution: creating a second object/Key-value-pair-thing for the 'http-enquiv' type.  Then you need to decide: do i want to add a `property` or a `http-enquiv`, but would be the same as with a select-form. Both solutions would have the possibility to expand.