@Override
    public void onSubscribe(Subscription subscription) {
        requireNonNull(subscription, "subscription");
        // This method is invoked in ctx.eventLoop.
        upstream = subscription;
        if (pendingRequests > 0) {
            subscription.request(pendingRequests);
            pendingRequests = 0;
        }
    }
Encapsulates the state of a single client call, writing messages from the client and reading responses
from the server, passing to business logic via {@link ClientCall.Listener}.
grpc/src/main/java/com/linecorp/armeria/internal/client/grpc/ArmeriaClientCall.java