@Override
    public Mono<HelloReply> helloError(Mono<HelloRequest> request) {
        return request.map(req -> {
            throw new AuthException(req.getName() + " is unauthenticated");
        });
    }
Throws an {@link AuthException}, and the exception will be handled
by {@link GrpcExceptionHandlerFunction}.
it/grpc/reactor/src/test/java/com/linecorp/armeria/grpc/reactor/TestServiceImpl.java