title: Replace woocommerce_add_order_item_meta hook in Woocommerce 3.4
body: <p>I have custom code use woocommerce_add_order_item_meta hook. But woocommerce 3.4.0 show error log: "woocommerce_add_order_item_meta is deprecated since version 3.0.0! Use woocommerce_new_order_item instead."
  How to fix it? Thank you very much. My code:</p>
  
  <pre><code>    // add data design to order
  function tshirt_order_meta_handler( $item_id, $values, $cart_item_key ) {
      if( WC()-&gt;session-&gt;__isset( $cart_item_key.'_designer' ) ) {
          wc_add_order_item_meta( $item_id, "custom_designer", WC()-&gt;session-&gt;get( $cart_item_key.'_designer') );
      }
  }
  add_action( 'woocommerce_add_order_item_meta', 'tshirt_order_meta_handler', 1, 3 );
  </code></pre>