@UnstableApi
    public VirtualHostBuilder successFunction(SuccessFunction successFunction) {
        this.successFunction = requireNonNull(successFunction, "successFunction");
        return this;
    }
Sets the {@link SuccessFunction} to define successful responses.
{@link MetricCollectingService} and {@link LoggingService} use this function.
core/src/main/java/com/linecorp/armeria/server/VirtualHostBuilder.java