docs: function getSampler(db, collectionName, opts, done) {
    db.admin().serverInfo(function(err, res) {
      if (err) return done(err);
      var supported = semver.gte(res.version, '3.1.6');
  
      debug('has native $sample?', supported);
      if (!supported) {
        return done(null, new ReservoirSampler(db, collectionName, opts));
      }
      return done(null, new NativeSampler(db, collectionName, opts));
    });
  }