code: function RootSpan(agent, span) {
    this.agent_ = agent;
    this.span_ = span;
    this.serializedTraceContext_ = agent.generateTraceContext(span, true);
  }
docs: An object that represents a single root span. It exposes functions for adding
  labels to or closing the span.
  @param {TraceAgent} agent The underlying trace agent object.
  @param {SpanData} span The internal data structure backing the root span.