In Mongo how do I rename a new added field in an aggregation?

When doing an aggregation I add a new field that has data from another object but in the last step I want to rename the field to something else(the original object I am getting data from)

What operator can I use to do that, of course, $replace doesn't work in this case

{
    $match: { _id: id },
},
{
    $addFields: {
        'userMetadata.active': '$user.active',
        'userMetadata.metadata': '$user.metadata',
    },
},
{ $unset: 'user' }
//Here I want to replace userMetadata with user

Thank you

Basically, I am looking for an alternative to adding this

{
    $addFields: {
       user: '$userMetadata',
    },
},
{ $unset: 'userMetadata' },

This solution works but I am not sure how performant it is over long lists

Update 2:
This seems to work, still more stages than I want

{
    $addFields: {
        'userMeta.active': '$user.active',
        'userMeta.metadata.name': '$user.metadata.name',
        'userMeta.metadata.url': '$user.metadata.url',
        user: '$$REMOVE',
    },
},
{
    $addFields: {
        user: '$userMeta',
        userMeta: '$$REMOVE',
    },
},

You can use this:

{
    $addFields: {
        'user': '$userMetadata',
        'userMetadata': '$$REMOVE'
    }
}

$unset also accepts an array:

{ $unset: [ 'user', '<more fields>', ... ] }

Or use $project, it removes all (apart from _id) fields which are not listed:

{
    $project: {
        'user': '$userMetadata',
        '_id': 0
    }
}