code: func NewTransport(next http.RoundTripper, cbs *Callbacks) http.RoundTripper {
  	// If next RoundTripper is not defined we'll use http.DefaultTransport.
  	if next == nil {
  		next = http.DefaultTransport
  	}
  
  	// If cbs is not defined we'll initilialize it with defaults.
  	if cbs == nil {
  		cbs = &Callbacks{}
  	}
  	// By default, path and query will be ignored.
  	if cbs.PathProcessor == nil {
  		cbs.PathProcessor = EliminatingProcessor
  	}
  	if cbs.QueryProcessor == nil {
  		cbs.QueryProcessor = EliminatingProcessor
  	}
  	// By default, status code is set as is.
  	if cbs.CodeProcessor == nil {
  		cbs.CodeProcessor = IntToStringProcessor
  	}
  
  	return &Transport{next: next, cbs: cbs}
  }
docs: // NewTransport takes a http.RoundTripper, wraps it with instrumentation and
  // returns it as a new http.RoundTripper. Optionally, It can receive a
  // collection of callbacks that process request path and query into a suitable
  // label value.