Nicholas Anderson

proposal: contrib/google.golang.org/grpc: make MDCarrier public

3

The [MDCarrier](https://github.com/DataDog/dd-trace-go/blob/v1/contrib/google.golang.org/internal/grpcutil/mdcarrier.go) is generally sufficient for non-custom metadata implementations. My proposal would be to either include the MDCarrier inside of the grpc folder(s), `grpc` and `grpc.v12`, or move the `grpcutil`...