code: function ReservoirSampler(db, collectionName, opts) {
    this.running = false;
    this.chunkSize = opts ? opts.chunkSize : undefined;
    BaseSampler.call(this, db, collectionName, opts);
  }
docs: A readable stream of sample of documents from a collection via
  [reservoir sampling](https://en.wikipedia.org/wiki/Reservoir_sampling).
  
  @param {mongodb.DB} db
  @param {String} collectionName to source from.
  @param {Object} opts
  @option {Object} query to refine possible samples [default: `{}`].
  @option {Number} size of the sample to capture [default: `5`].
  @option {Object} fields to return for each document [default: `null`].
  @option {Number} chunkSize for chunked $in queries [default: `1000`].
  @api public