default EventExecutor defaultSubscriberExecutor() {
        final EventLoop eventExecutor = RequestContext.mapCurrent(RequestContext::eventLoop,
                                                                  CommonPools.workerGroup()::next);
        assert eventExecutor != null;
        return eventExecutor;
    }
Returns the default {@link EventExecutor} which will be used when a user subscribes using
{@link #subscribe(Subscriber)}, {@link #subscribe(Subscriber, SubscriptionOption...)}.

<p>Please note that if this method is called multiple times, the returned {@link EventExecutor}s can be
different depending on this {@link StreamMessage} implementation.
core/src/main/java/com/linecorp/armeria/common/stream/StreamMessage.java