@Override
    public void onSubscribe(Subscription subscription) {
        requireNonNull(subscription, "subscription");
        // 'subscribe()' only happens in the constructor of ArmeriaServerCall.
        upstream = subscription;
        if (pendingRequests > 0) {
            upstream.request(pendingRequests);
            pendingRequests = 0;
        }
    }
Encapsulates the state of a single server call, reading messages from the client, passing to business logic
via {@link ServerCall.Listener}, and writing messages passed back to the response.
grpc/src/main/java/com/linecorp/armeria/server/grpc/StreamingServerCall.java