Trailer metadata - repeated values only supported with "trailers-only" responses

If a gRPC response sets multiple values for the same trailer key, split across multiple lines, all of the values are available (via the `metadata` property of the status) if the response arrives as a "trailers-only" response (meaning that the trailers are all encoded as HTTP headers and there is no body).

But if the trailer metadata arrives in the body, as the final special message in the stream, only the final value is made available, as if the others were never even present. Most likely, a simple map (key -> singular string value) is being used to process the final message, so later values are incorrectly overwriting earlier ones.