title: Filter function score with Nest
body: <p>I am trying to build the following query using using the DSL syntax:</p>
  
  <pre><code>    GET /_search
      {
        "query": {
          "function_score": {
            "filter": { 
              "term": { "city": "Barcelona" }
            },
            "functions": [ 
              {
                "filter": { "term": { "features": "wifi" }}, 
                "weight": 1
              },
              {
                "filter": { "term": { "features": "garden" }}, 
                "weight": 1
              },
              {
                "filter": { "term": { "features": "pool" }}, 
                "weight": 2 
              }
            ],
            "score_mode": "sum", 
          }
        }
      }
  </code></pre>
  
  <p>However it seems there's no <code>filter</code> option in the function_score when using the NEST client. I can confirm the query works in elastic.</p>