public DnsCacheBuilder cacheSpec(String cacheSpec) {
        this.cacheSpec = cacheSpec;
        return this;
    }
Sets the {@linkplain CaffeineSpec Caffeine specification string}.
If not specified, {@link Flags#dnsCacheSpec()} is used by default.
core/src/main/java/com/linecorp/armeria/client/DnsCacheBuilder.java