getMetadata() {
    return this._transport.getMetadata();
  }
@returns {Promise<{ info: Object, metadata: Metadata }>} A promise that is
  resolved with an {Object} that has `info` and `metadata` properties.
  `info` is an {Object} filled with anything available in the information
  dictionary and similarly `metadata` is a {Metadata} object with
  information from the metadata section of the PDF.