public SELF timestampFunction(Function<? super T, @Nullable Long> timestampFunction) {
        requireNonNull(timestampFunction, "timestampFunction");
        //noinspection unchecked
        this.timestampFunction = (Function<T, Long>) timestampFunction;
        return self();
    }
Sets the timestamp function to use to get the creation time of the given candidate.
The timestamp is used to calculate the ramp up weight of the candidate.
If {@code null} is returned or the timestamp function is not set, the timestamp is set to the current
time when the candidate is added.
core/src/main/java/com/linecorp/armeria/common/loadbalancer/AbstractRampingUpLoadBalancerBuilder.java