Is it possible to rename _id field after mongo's group aggregation?

You can achieve this by adding a $project stage at the end of your pipeline like this :

{ $project: {  
      _id: 0,
      name: "$_id",
      count: 1,
      sum: 1
   }
}

try it online: mongoplayground.net/p/QpVyh-0I-bP

From mongo v3.4 you could use $addFields in conjunction with $project to avoid to write all the fields in $project that could be very tedious.

This happen in $project because if you include specifically a field, the other fields will be automatically excluded.

Example:

{ 
  $addFields: { my_new_id_name: "$_id" }
},
{
  $project: { _id: 0 }
}

 db.report.aggregate(   
{     
$group: {_id: '$name'} 
},
{
$project:{
  name:"$_id",
 _id:false} }
 )