@Override
        int index() {
            return index;
        }
Index in the binary heap {@link HeapBasedEventLoopState#entries}.
Updated by {@link HeapBasedEventLoopState#swap(int, int)} after
{@link #activeRequests()} is updated by {@link HeapBasedEventLoopState#acquire()} and
{@link HeapBasedEventLoopState#release(AbstractEventLoopEntry)}.
core/src/main/java/com/linecorp/armeria/client/HeapBasedEventLoopState.java