code: function NativeSampler(db, collectionName, opts) {
    BaseSampler.call(this, db, collectionName, opts);
    this.running = false;
  }
docs: A readable stream of sample of documents from a collection using the
  `$sample` aggregation operator.
  
  @param {mongodb.DB} db
  @param {String} collectionName to source from.
  @param {Object} opts
  @option {Object} query to refine possible samples [default: `{}`].
  @option {Array} fields to only return certain fields [default: null]
  @option {Number} size of the sample to capture [default: `5`].
  @option {Boolean} return document results as raw BSON buffers [default: `false`].
  @api public