Unable to add multiple subscriptions

[NickLarsenNZ]

There is currently no way to add additional subscriptions. ListTopicSubscriptionResponse::topic(...) is hard coded to a vector of one subscription: https://docs.rs/dapr/0.9.0/src/dapr/appcallback.rs.html#42

Originally posted by @NickLarsenNZ in #80 (comment)

---

Other SDKs seem to be able to do it (I have seen it in action with .Net), for example dapr/python-sdk:

https://github.com/dapr/python-sdk/blob/1aab2cb1e06b32899183a285eab5c975408d05b2/ext/dapr-ext-grpc/dapr/ext/grpc/app.py#L137-L140

[NickLarsenNZ]

Actually, it looks like I can if I avoid using the topic method, and instead build the ListTopicSubscriptionsResponse struct from scratch like so:

    async fn list_topic_subscriptions(
        &self,
        _request: Request<()>,
    ) -> Result<Response<ListTopicSubscriptionsResponse>, Status> {
        // This commented out section only allows a single subscription with no ability to extend.
        // let list_subscriptions =
        //     ListTopicSubscriptionsResponse::topic(self.pubsub_name.clone(), self.topic.clone());


        let empty_metadata = None;

        let list_subscriptions = ListTopicSubscriptionsResponse {
            subscriptions: vec![
                TopicSubscription::new("pubsub".to_owned(), "a".to_owned(), empty_metadata.clone()),
                TopicSubscription::new("pubsub".to_owned(), "b".to_owned(), empty_metadata.clone()),
            ],
        };

        Ok(Response::new(list_subscriptions))
    }

This appears to work, based on the daprd logs:

time="2022-09-30T22:56:55.85175101Z" level=info msg="app is subscribed to the following topics: [a b] through pubsub=pubsub" app_id=job-scheduler instance=a1f7de26e44f scope=dapr.runtime type=log ver=edge

[NickLarsenNZ]

It does appear to silently fail if you create two instances of the Dapr AppCallbackServers, like this which I had originally tried:

    Server::builder()
        .trace_fn(|_| tracing::info_span!("tonic server"))
        .add_service(AppCallbackServer::new(job_runner::JobRunner::new(
            pubsub.as_str(),
            topic.as_str(),
        )))
        .add_service(AppCallbackServer::new(job_runner::JobRunner::new(
            pubsub.as_str(),
            "another.topic",
        )))
        .serve(addr)
        .await?;

[mikeee]

#87 (comment)
I think this can be addressed for now with the above merged. Let me know any thoughts otherwise :)