    static void extractAndInsertUserName(unordered_set<UserName>* existingUsers,
                                         const BSONObj& userObj) {
        UserName userName = extractUserNameFromBSON(userObj);
        existingUsers->insert(userName);
    }
    static void extractAndInsertUserName(unordered_set<UserName>* existingUsers,
                                         const BSONObj& userObj) {
        UserName userName = extractUserNameFromBSON(userObj);
        existingUsers->insert(userName);
    }