code: function getIndexSizes(done, results) {
    var client = results.client;
    var ns = mongodbNS(results.namespace);
    client
      .db(ns.database)
      .collection(ns.collection)
      .stats(function(err, res) {
        if (err) {
          if (isNotAuthorizedError(err)) {
            debug(
              'Not authorized to get collection stats.  Returning default for indexSizes {}.'
            );
            return done(null, {});
          }
          return done(err);
        }
  
        res = _.mapValues(res.indexSizes, function(size) {
          return { size: size };
        });
        done(null, res);
      });
  }
docs: get index sizes via `db.collection.stats()` (`indexSizes` field)
  @param  {Function} done       callback
  @param  {object}   results    results from async.auto