title: Filtering specific product ID in WooCommerce Order items
body: <p>I have a function that is generating a unique string. This is currently working. However it is running for all products that are sold at checkout. I would like the following function to only run if a specific product is in the order Items. </p>
  
  <p>Here is what I have so far:</p>
  
  <pre><code>add_action( 'woocommerce_order_status_processing', 'add_unique_id' );
  
  function add_unique_id($order_id) {
      $order = new WC_Order( $order_id );
      $items = $order-&gt;get_items(); 
      foreach ($items as $item_id =&gt; $product ) {
          $str = "";
          $length= 45;
  $characters = array_merge(range('A','Z'), range('a','z'), range('0','9'));
  $max = count($characters) - 1;
      for ($i = 0; $i &lt; $length; $i++) {
      $rand = mt_rand(0, $max);
      $str .= $characters[$rand];
      }
        wc_add_order_item_meta($item_id, 'Member Number', $str);
      }
  }
  </code></pre>