static <T> WeightTransition<T> linear() {
        //noinspection unchecked
        return (WeightTransition<T>) LinearWeightTransition.INSTANCE;
    }
Returns the {@link WeightTransition} which returns the gradually increased weight as the current
step increases.
core/src/main/java/com/linecorp/armeria/common/loadbalancer/WeightTransition.java