private RpcService decorate(RpcService service) {
        if (decoratorFunction != null) {
            return service.decorate(decoratorFunction);
        }
        return service;
    }
A {@code Function<? super RpcService, ? extends RpcService>} to decorate the {@link RpcService}.
thrift/thrift0.13/src/main/java/com/linecorp/armeria/server/thrift/THttpServiceBuilder.java