title: fluentd remove keys dynamically
body: <p>I need to remove keys from json messages, pretty easy with <strong>record_transformer</strong> and <strong>keep_keys</strong> or <strong>remove_keys</strong>.
  But the keys I want to remove are values from a specific key (ex: filter)</p>
  <p><em>input like:</em></p>
  <p><code>{&quot;message&quot;:&quot;hello world!&quot;, &quot;key1&quot;:&quot;test1&quot;, &quot;key2&quot;=&quot;test2&quot;, &quot;key3&quot;=&quot;test3&quot;, &quot;**filter**&quot;=&quot;message,key3&quot;}</code></p>
  <p><em>want transformed into:</em></p>
  <p><code>{&quot;message&quot;:&quot;hello world!&quot;, &quot;key3&quot;=&quot;test3&quot;}</code></p>
  <p>I want the keep_keys parameter been dynamic for each messages.
  How this can be achieve ? with just config or with plugin modification ?</p>
  <p>Any suggestions ?</p>