protected final void produced(long n) {
        assert executor.inEventLoop();
        newProduced = LongMath.saturatedAdd(newProduced, n);
        drain();
    }
Indicate how many items were produced from the current subscription
before switching to the next subscription.
@param n the number of items produced, positive
core/src/main/java/com/linecorp/armeria/internal/common/stream/SubscriptionArbiter.java