code: func NewTransport(tracer *zipkin.Tracer, options ...TransportOption) (http.RoundTripper, error) {
  	if tracer == nil {
  		return nil, ErrValidTracerRequired
  	}
  
  	t := &transport{
  		tracer:     tracer,
  		rt:         http.DefaultTransport,
  		httpTrace:  false,
  		errHandler: defaultErrHandler,
  	}
  
  	for _, option := range options {
  		option(t)
  	}
  
  	return t, nil
  }
docs: // NewTransport returns a new Zipkin instrumented http RoundTripper which can be
  // used with a standard library http Client.