title: Include all existing fields and add new fields to document
body: <p>I would like to define a $project aggregation stage where I can instruct it to add a new field and include all existing fields, without having to list all the existing fields.</p>
  
  <p>My document looks like this, with many fields:</p>
  
  <pre class="lang-json prettyprint-override"><code>{
      obj: {
          obj_field1: "hi",
          obj_field2: "hi2"
      },
      field1: "a",
      field2: "b",
      ...
      field26: "z"
  }
  </code></pre>
  
  <p>I want to make an aggregation operation like this:</p>
  
  <pre class="lang-json prettyprint-override"><code>[
      {
          $project: {
              custom_field: "$obj.obj_field1",
              //the next part is that I don't want to do
              field1: 1,
              field2: 1,
              ...
              field26: 1
          }
      },
      ... //group, match, and whatever...
  ]
  </code></pre>
  
  <p>Is there something like an "include all fields" keyword that I can use in this case, or some other way to avoid having to list every field separately?</p>