void decrementConcurrency() {
            if (concurrencyDecremented.compareAndSet(false, true)) {
                filter.decrementConcurrency();
            }
        }
This is typically set by a filter when wanting to reject a request and also reduce load on the server by
 not processing anymore filterChain
zuul-core/src/main/java/com/netflix/zuul/netty/filter/BaseZuulFilterRunner.java