code: public void startSpanWithContext(SpanBuilder spanBuilder, SpanContext context, String id) {
          if (context != null) {
              spanBuilder.asChildOf(context);
          }
  
          doStartSpan(spanBuilder, id);
      }
docs: This is a convenience method for situations where we don't know
  if a parent span is available. If we try to add a childOf relationship
  to a null context, it would cause a null pointer exception.
  
  The optional id is associated with the started span.
  
  @param spanBuilder The span builder
  @param context The span context
  @param id The optional id to associate with the span