title: Add extra meta for orders in Woocommerce
body: <p>I'm creating a custom plugin for my website.</p>
  
  <p>In some part of this plugin I need to store extra meta in <code>wp_postmeta</code> for each orders.</p>
  
  <p>I added this in my plugin's class:</p>
  
  <pre><code>add_action ('woocommerce_before_checkout_process', array( &amp;$this, 'add_item_meta', 10, 2) );
  </code></pre>
  
  <p>And this is <code>add_item_meta()</code> function:</p>
  
  <pre><code>function add_item_meta( $item_id, $values ) {
    wc_add_order_item_meta($item_id, '_has_event', 'yes' );
  }
  </code></pre>
  
  <p>This function is not complete, but nothing happens with this codes; I think I need to use another hook but I can't find a proper one.</p>
  
  <p>Does anyone know anything about this?</p>
  
  <p>I also have another problem with <code>$item_id</code>: this is woocommerce global variable but I can't see it in my plugin!</p>
  
  <p>I mean I don't have access to this variable from my plugin or something like this!</p>