Use Proto instead of json in go gRPC client

Hello folks,
i am using go gRPC client and i am using the content type right now as json and i want to start using proto instead of json but when i checked the code i found out that the store save the data in binary format regardless the content type can you advise please what is the content type used for and how i start using proto instead of json ?

The content type system meta is mainly for you, so you can deserialise it on the other size. We do check for application/json if you use custom projections (server-side) as they only work with JSON payloads. If you aren’t using them, specify the content type that you want.