In a Change Stream pipeline, we would like to refer to updated fields in
embedded documents, e.g. updateDescription: { updatedFields: { "a.b.c": "Foo" } }.

But field path expressions cannot refer to such fields because they contain
dots, and field path expressions lack an escape-character; for example like
"$updateDescription.updatedFields.a..b..c".

A workaround for some scenarios is to use the full-document option to have MongoDB
lookup the document and include it in the result, but this incurs a performance-cost
and may return the document in a different state; both of which make the workaround unsuitable for our use-case.

{
    "_id": { "_data": /**/ },
    "operationType": "update",
    "ns": { "db": "test", "coll": "test" },
    "documentKey": { "_id": /**/ },
    "updateDescription": {
       "updatedFields": { "a.b.c": "Foo" },
       "removedFields": []
    }
}

changeStream = db.test.watch(
    [
        {
            $project: {
                c: "$updateDescription.updatedFields.a.b.c"
            }
        }
    ]
);