title: Elasticsearch : aggregation "existing" fields
body: <p>I'm quite new to ElasticSearch aggregations. I want to be able to count how many documents are retrieved with a not null field.</p>
  
  <p>Here's what I do to count how many documents don't have a <code>name</code> field.</p>
  
  <pre><code>{
    "size": 3,
    "query": {
      "query_string": {
        "query": "martin"
      }
    },
    "aggs": {
      "results_without_mb_id": {
        "missing": {
          "field": "name"
        }
      }
    }
  }
  </code></pre>
  
  <p>It works but I want to do the exact opposite. Is there an <code>existing</code> aggregation?</p>