code: function ChildSpan(agent, span) {
    this.agent_ = agent;
    this.span_ = span;
    this.serializedTraceContext_ = agent.generateTraceContext(span, true);
  }
docs: This file describes an interface for third-party plugins to enable tracing
  for arbitrary modules.
   
  An object that represents a single child 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 child span.