title: Mongodb count how often a value exist in collection
body: <p>I try to write a query, which returns how often the value <code>keyword4</code> appears in my collection for the field <code>label</code></p>
  
  <pre><code>{
      "_id" : ObjectId("54071d4312bf9ee0d67f7cf8"),
      "id" : "2",
      "list" : [ 
          {
              "id" : 60,
              "label" : "keyword3",
              "pos" : 1
          }, 
          {
              "id" : 89,
              "label" : "keyword4",
              "pos" : 2
          }, 
          {
              "id" : 49,
              "label" : "keyword4",
              "pos" : 3
          }
          ]
  },
  {
      "_id" : ObjectId("5407sfkjsvnsvree9ee0d67f7cf8"),
      "id" : "3",
      "list" : [ 
          {
              "id" : 60,
              "label" : "keyword3",
              "pos" : 1
          }, 
          {
              "id" : 89,
              "label" : "keyword4",
              "pos" : 2
          }, 
          {
              "id" : 49,
              "label" : "keyword4",
              "pos" : 3
          }
          ]
  },
  ...
  </code></pre>
  
  <p>Thanks.</p>