Provide the ability to set and retrieve custom metadata (key, value strng pairs) to each document and provide query capability over it.
For example select value from m where key ="myKey".
This is already supported. Azure Cosmos DB supports setting any user-defined property. Because it is schema-less and automatically indexes all properties, this is efficient and needs no additional hooks from the API.
Please email us at firstname.lastname@example.org if you need help with this.