Devil's advocate incoming :-D
What would you do in case the payload format doesn't allow for wrapping in an envelope because it cannot be extended like that, such as a paginated slice of a CSV data dump? Or a chunk of a binary data stream?
This is now veering away from web api design, where you would use json/xml over http in 99% of cases. The answer is, it depends.
For csv, I would probably not include metadata at all, but require recipients to obtain it using a separate method. For binary data, you would probably get metadata as part of the binary format itself. But yes, maybe also as headers in the transport protocol.
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