code: protected void initTopLevelState(APMSpan topSpan, TraceRecorder recorder, ContextSampler sampler) {
          nodeBuilder = new NodeBuilder();
          traceContext = new TraceContext(topSpan, nodeBuilder, recorder, sampler);
      }
docs: This method initialises the node builder and trace context for a top level
  trace fragment.
  
  @param topSpan  The top level span in the trace
  @param recorder The trace recorder
  @param sampler The sampler