code: function createRootSpan_(api, options, skipFrames) {
    options = options || {};
    // If the options object passed in has the getTraceContext field set,
    // try to retrieve the header field containing incoming trace metadata.
    var incomingTraceContext;
    if (is.string(options.traceContext)) {
      incomingTraceContext = api.agent_.parseContextFromHeader(options.traceContext);
    }
    incomingTraceContext = incomingTraceContext || {};
    if (!api.agent_.shouldTrace(options, incomingTraceContext.options)) {
      cls.setRootContext(nullSpan);
      return null;
    }
    var rootContext = api.agent_.createRootSpanData(options.name,
      incomingTraceContext.traceId,
      incomingTraceContext.spanId,
      skipFrames + 1);
    return new RootSpan(api.agent_, rootContext);
  }
docs: Module-private functions