title: How to exclude terms in nested composite aggregation
body: <p>I am using composite aggregation on nested fields in elasticsearch but I want to exclude some terms from the result.</p>
  
  <p>This aggregation is working:</p>
  
  <pre><code>{
    "size": 0,
    "geo": {
      "communication": {
        "nested": {
          "path": "geo"
        },
        "aggs": {
          "table": {
            "composite": {
              "size": 1000,
              "sources": [
                {"stk1": {"terms": {"field": "geo.src"}}},
                {"stk2": {"terms": {"field": "geo.dest"}}}
              ]
            }
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>But I want to exclude some terms from stk2,</p>
  
  <pre><code>{
    "size": 0,
    "aggs": {
      "geo": {
        "nested": {
          "path": "geo"
        },
        "aggs": {
          "table": {
            "composite": {
              "size": 1000,
              "sources": [
                {"stk1": {"terms": {"field": "geo.src"}}},
                {"stk2": {"terms": {"field": "geo.dest", "exclude":"cancel"}}}
              ]
            }
          }
        }
      }
    }
  }
  
  </code></pre>
  
  <p>The above query is not working.</p>
  
  <p>UPDATE 1: The result should omit only the array elements and not the entire document containing "cancel".</p>
  
  <p>I am using elastic v6.7</p>