title: Logstash add date field to logs
body: <p>My application produces logs, without a timestamp. 
  Is there a way in logstash to append timestamp to the logs on processing</p>
  
  <p>something like,</p>
  
  <pre><code>mutate {
      add_field =&gt; { "timestamp" =&gt; "%{date}" }
    } 
  </code></pre>