title: Aggregation Filter in ES DSL
body: <p>I have the following aggregation filter:</p>
  
  <p>"aggs": {
      "agg_name" : {
        "filter" : { "exists": { "field": "filed_name" } }
      }
    }
  }</p>
  
  <p>But I want to write it cleaner using <a href="https://elasticsearch-dsl.readthedocs.io/en/latest/search_dsl.html" rel="nofollow noreferrer">elasticsearch-dsl</a>. This is the best I could do:</p>
  
  <p>aggs.bucket('agg_name', {"filter": {"exists": {"field": "field_name"}}})</p>
  
  <p>But I know there is a cleaner way. Some failed attempts:</p>
  
  <ul>
  <li>aggs.bucket('agg_name', A('filter', type='exists',
  field='field_name'))</li>
  <li>aggs.bucket('agg_name', A('filter','exists',
  field='field_name'))</li>
  <li>aggs.bucket('agg_name', A().filter('exists',
  field='field_name'))</li>
  <li>aggs.bucket('agg_name').filter('exists',
  field='field_name')</li>
  </ul>
  
  <p>I'm currently not able to get the right information out of the documentation. Is it possible to write it more DSL-like?</p>