title: flush timeoutError in fluentd
body: <p>I am running <code>fluentd</code> with following config:</p>
  
  <pre><code>&lt;source&gt;
      @type forward
      port 24224
  &lt;/source&gt;
  &lt;filter docker.**&gt;
      @type concat
      key msg
      stream_identity_key uuid
      multiline_start_regexp /url:/
      multiline_end_regexp /.* ---$/
  &lt;/filter&gt;
  &lt;match docker.**&gt;
      @type copy
      &lt;store&gt;
          @type stdout
      &lt;/store&gt;
  &lt;/match&gt;
  </code></pre>
  
  <p>I am getting below <code>Fluent::ConcatFilter::TimeoutError</code> when some <code>event</code> occurs:</p>
  
  <pre><code>2016-05-11 15:22:38 +0530 [warn]: dump an error event: error_class=Fluent::ConcatFilter::TimeoutError error="Timeout flush: docker.containers:67deabd67c04199d" tag="docker.containers" time=1462960358 record={"msg"=&gt;"url: /get/image.jpg\nid: 83110292-24a4-62a8-c155-ebc0f9bbb976"}
  2016-05-11 15:22:38 +0530 [info]: Timeout flush: docker.containers:67deabd67c04199d
  </code></pre>
  
  <p>Could anyone please tell me why this error is coming and how to resolve it</p>
  
  <p>Thanks</p>