Function<? super HttpService, ? extends HttpService> decorator() {
        if (decorator == null) {
            return Function.identity();
        }
        return decorator;
    }
A default implementation of {@link ServiceConfigSetters} that stores service related settings
and provides a method {@link DefaultServiceConfigSetters#toServiceConfigBuilder(Route, String, HttpService)}
to build {@link ServiceConfigBuilder}.
core/src/main/java/com/linecorp/armeria/server/DefaultServiceConfigSetters.java