@Nullable
    private static Message marshallerPrototype(Marshaller<?> marshaller) {
        if (marshaller instanceof PrototypeMarshaller) {
            final Object prototype = ((PrototypeMarshaller<?>) marshaller).getMessagePrototype();
            if (prototype instanceof Message) {
                return (Message) prototype;
            }
        }
        return null;
    }
Returns a {@link MessageMarshaller} with the request/response {@link Message}s of all the {@code methods}
registered.
grpc/src/main/java/com/linecorp/armeria/common/grpc/GrpcJsonUtil.java