title: Mongoose 6 $group with Typescript -> Property '_id' is incompatible with index signature
body: <p>I am migrating from js to ts and also from mongoose 5.8 to 6.1
  The following code was working fine earlier</p>
  <pre><code>let getActionsTakenByApp = (store_url: string) =&gt; {
      return AppAction.aggregate([
          {
              $match: { store_url }
          },
          {
              $group: {
                  _id: {
                      'action_type': '$action_type'
                  },
                  count: {
                      $sum: 1
                  }
              }
          }
      ]);
  }
  </code></pre>
  <p>But now typescript is now showing this error.</p>
  <pre><code>Type '{ _id: { action_type: string; }; count: { $sum: number; }; }' is not assignable to type '{ [key: string]: { $count?: any; $accumulator?: any; $addToSet?: any; $avg?: any; $first?: any; $last?: any; $max?: any; $mergeObjects?: any; $min?: any; $push?: any; $stdDevPop?: any; $stdDevSamp?: any; $sum?: any; }; _id: any; }'.   Property '_id' is incompatible with index signature.
      Type '{ action_type: string; }' is not assignable to type '{ $count?: any; $accumulator?: any; $addToSet?: any; $avg?: any; $first?: any; $last?: any; $max?: any; $mergeObjects?: any; $min?: any; $push?: any; $stdDevPop?: any; $stdDevSamp?: any; $sum?: any; }'.
        Object literal may only specify known properties, and ''action_type'' does not exist in type '{ $count?: any; $accumulator?: any; $addToSet?: any; $avg?: any; $first?: any; $last?: any; $max?: any; $mergeObjects?: any; $min?: any; $push?: any; $stdDevPop?: any; $stdDevSamp?: any; $sum?: any; }'.ts(2322)
  </code></pre>
  <p>I'm still new to typescript and trying to figure out what's the issue. Any help would be appreciated! Thank you!!</p>