Metafields by Namespace or Key in Admin REST API: Does Not Work

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https://shopify.dev/docs/admin-api/rest/reference/metafield?api%5Bversion%5D=2021-01#index-2021-01

Retrieves a list of metafields that belong to a resource

But then proceeds to list parameters, none of which are for a resource. 

It does list namespace and key, but using them does not work. Nothing is returned: 

https://MYSHOP.myshopify.com/admin/api/2021-01/metafields.json?namespace=x

Does not work

https://MYSHOP.myshopify.com/admin/api/2021-01/metafields.json?metafield[namespace]=x

Does not work. 

Neither do similar requests using key.

How can one retrieve metafields by namespace or key? 

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Reading the docs again it says that: https://MYSHOP.myshopify.com/admin/metafields.json is the URL for metafields associated with a shop. This was the problem. 

So the question becomes: is there a way to get metafields across all resources by namespace?