Add new column to woocommerce product grid, add custom fields value

functions.php

add_filter( 'manage_edit-product_columns', 'admin_add_in_promo', 10, 2 );
function admin_add_in_promo($columns){
   //remove column
   //unset( $columns['tags'] );
   //add column
   $columns['in_promo'] = __( 'Promo'); 
   return $columns;
}
add_action( 'manage_posts_custom_column', 'admin_show_in_promo', 10 );
function admin_show_in_promo( $column) {
  global $post;
    if ( $column == 'in_promo' ) {
        if( function_exists('get_product') ){
          $product = get_product( $post->ID );
            if( $product->is_type( 'variable' ) ){
              $available_variations = $product->get_available_variations();
                foreach($available_variations as $k) {
                  $_in_promo = get_post_meta($k[variation_id], '_in_promo', true);
                    if($_in_promo == yes) {
                      $modulo = $k[attributes][attribute_pa_modulo];
                      echo $modulo . '<br />';
                      $colore = $k[attributes][attribute_pa_colore];
                      echo $colore . '<br />';
                      echo 'Fino al:<br />';
                      $_special_price_to_date = get_post_meta($k[variation_id], '_special_price_to_date', true);
                      echo $_special_price_to_date;
                    }
                } 
            }
        }
    }
}