code: func NewClient(next *http.Client, cbs *Callbacks) *http.Client {
  	// If next client is not defined we'll use http.DefaultClient.
  	if next == nil {
  		next = http.DefaultClient
  	}
  
  	next.Transport = NewTransport(next.Transport, cbs)
  
  	return next
  }
docs: // NewClient takes a *http.Client and returns a *http.Client that has its
  // RoundTripper wrapped with instrumentation. Optionally, It can receive a
  // collection of callbacks that process request path and query into a suitable
  // label value.