Macro adding metadata to incremental model

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1

Hello,
I would like to write macro which will add metadata to the given model (can be incremental). I started with this:

{% macro add_metadata() %}
    {% if is_incremental() %}
        , current_timestamp() as updated_at
        , session_user() as updated_by
    {% else %}
        , current_timestamp() as created_at
        , session_user () as created_by
        , null as updated_at
        , null as updated_by
    {% endif %}
{% endmacro %}

but the problem is that when I have staging model like this for example:

SELECT 1 AS id, 'Alice' AS name, 100 AS score
UNION ALL
SELECT 2, 'Bob', 200
UNION ALL
SELECT 3, 'Charlie', 300

and my target model looks like this:

{{ config(
    materialized='incremental',
    unique_key='id'
) }}

WITH source_data AS (
    SELECT s.*
    {{ add_metadata()}} 
    
    FROM {{ ref('testing_metadata') }} s
)

SELECT *
FROM source_data

I am always receiving error that fields created_at and created_by do not exist because it tries to take it from staging model. How to improve this macro to change only updated_by and updated_at fields when I will be running the model for 2nd and next times and fields created_at and created_by will remain the same?