private void addPendingFuture(ListeningFuture future) {
        lock.lock();
        try {
            pendingFutures.add(future);
        } finally {
            lock.unlock();
        }
    }
Triggers pending futures to try and select a value by calling {@link #selectNow(ClientRequestContext)}.
core/src/main/java/com/linecorp/armeria/internal/client/AbstractAsyncSelector.java