private void request0(int numMessages) {
        if (upstream == null) {
            pendingRequests += numMessages;
        } else {
            upstream.request(numMessages);
        }
    }
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