 information incoming or outgoing
    headers, cookies, session or request attributes and special
    timestamp formats.
    It is modeled after the
    <a href="http://httpd.apache.org/">Apache HTTP Server</a> log configuration
    syntax. Each of them can be used multiple times with different <code>xxx</code> keys:
    </p>
    <ul>
    <li><b><code>%{xxx}i</code></b> write value of incoming header with name <code>xxx</code></li>
    <li><b><code>%{xxx}o</code></b> write value of outgoing header with name