code: function TraceWriter(logger, options) {
    options = options || {};
  
    EventEmitter.call(this);
  
    /** @private */
    this.logger_ = logger;
  
    /** @private */
    this.config_ = options;
  
    /** @private {Array<string>} stringified traces to be published */
    this.buffer_ = [];
  
    /** @private {Object} default labels to be attached to written spans */
    this.defaultLabels_ = {};
  
    /** @private {Boolean} whether the trace writer is active */
    this.isActive = true;
  }
docs: Creates a basic trace writer.
  @param {!Logger} logger The Trace Agent's logger object.
  @param {Object} config A config object containing information about
  authorization credentials.
  @constructor