title: Woocommerce REST API extending order response
body: <p>I'm looking for a way to extend the wc-api/vX/orders/ reponse. I've added multiple custom fields to the checkout (for eg: relation number, delivery date etc). These meta are saved within the order (wp_postmeta table). But why are they not returned with the api?</p>
  
  <p>Normally you can extend the api response with some code like:</p>
  
  <pre><code>add_action( 'rest_api_init', 'custom_register_api_fields' );
  
  function custom_register_api_fields() {
      register_rest_field( 'shop_order','relation_number',
      array(
          'get_callback'    =&gt; 'custom_api_meta_callback',
          'update_callback' =&gt; null,
          'schema'          =&gt; null,
      )
      );
  }
  
  /**
  *
  * @param array $object Details of current post.
  * @param string $field_name Name of field.
  * @param WP_REST_Request $request Current request
  *
  * @return mixed
  */
  
  function custom_api_meta_callback( $object, $field_name, $request ) {
   return get_post_meta( $object[ 'id' ], $field_name, true );
  }
  </code></pre>
  
  <p>But when I test the response (with Postman and the php lib), my-website.co/wc-api/v2/orders the custom meta are not visible. </p>
  
  <p>Is there a way to register api fields for the wc-api?</p>
  
  <p>Tnx!</p>