code: func NewRecordedTrace(opName string, callback func(sp basictracer.RawSpan)) opentracing.Span {
  	tr := basictracer.New(CallbackRecorder(callback))
  	opentracing.SetGlobalTracer(tr)
  	sp := tr.StartSpan(opName)
  	sp.SetBaggageItem(TiDBTrace, "1")
  	return sp
  }
docs: // NewRecordedTrace returns a Span which records directly via the specified
  // callback.