code: function getIndexStats(done, results) {
    var client = results.client;
    var ns = mongodbNS(results.namespace);
    var pipeline = [
      { $indexStats: {} },
      {
        $project: {
          name: 1,
          usageHost: '$host',
          usageCount: '$accesses.ops',
          usageSince: '$accesses.since'
        }
      }
    ];
    var collection = client.db(ns.database).collection(ns.collection);
    collection.aggregate(pipeline, { cursor: {} }).toArray(function(err, res) {
      if (err) {
        if (isNotAuthorizedError(err)) {
          /**
           * In the 3.2 server, `readWriteAnyDatabase@admin` does not grant sufficient privileges for $indexStats.
           * The `clusterMonitor` role is required to run $indexStats.
           * @see https://jira.mongodb.org/browse/INT-1520
           */
          return done(null, {});
        }
  
        if (err.message.match(/Unrecognized pipeline stage name/)) {
          // $indexStats not yet supported, return empty document
          return done(null, {});
        }
        done(err);
      }
      res = _.mapKeys(res, function(stat) {
        return stat.name;
      });
      done(null, res);
    });
  }
docs: get index statistics via `db.collection.aggregate({$indexStats: {}})`
  @param  {Function} done       callback
  @param  {object}   results    results from async.auto