title: Convert String to Int on AWS DocumentDB
body: <p>I am currently trying to write a metabase question off AWS Document DB and I am running into an issue where I need to convert a string to an integer.  Unfortunately, it seems like aws documentdb does not support $toInt and I am not sure how to get around it.  Here is the query:</p>
  
  <pre><code>[
      {"$match": { 
          "metaData.fileSize" : {"$exists": true}
      }},
      {"$project": { 
          "file_size" : "$metaData.fileSize",
          "timestamp": 1,
          "past7Days": 
          { "$subtract": 
                  [ ISODate(), 604800000]
          }
      }},
      {"$project": { 
          "file_size" : 1,
          "timestamp": 1,
          "dayofweek": {"$dayOfWeek":["$timestamp"]},
          "past7DaysComp": 
          { "$subtract": 
                  [ "$timestamp", "$past7Days"]
          }
      }},
      {"$group" : 
              {   
                  "_id" : {"dayofweek" : "$dayofweek"},
                  "size": {"$avg" : "$file_size"}
              }
      }
  ]
  </code></pre>
  
  <p>The group returns nothing for size since it is not of a numeric type.  Any ideas how to convert file_size to integer or double or float?</p>