title: dynamic URL parameters with logstash http-poller
body: <p>I would like to use logstash's http-poller input to consume a webservice returning an array of json documents to load data into elasticsearch. In order to do this, the webservice GET URL needs to be updated with URL query parameters between requests.</p>
  
  <p>For example, the web service takes a date range specified as start and end parameters and limits responses to 1000 results. When first loading data, I would like logstash to continually call the web service with the start time getting set to the latest timestamp from the previous request until all data has been loaded. </p>
  
  <p>To clarify, the first HTTP GET request should look like <code>webserviceurl?start=2015-01-01T00:00:00</code>. Assume the latest document in the response has the timestamp 2015-01-01T21:55:34. Then the next HTTP GET request should look like <code>webserviceurl?start=2015-01-01T21:55:34</code></p>
  
  <p>Is this possible with logstash and the http-poller input?   </p>