code: func (c *Config) Wrap(fn WrapperFunc) {
  	c.WrapTransport = Wrappers(c.WrapTransport, fn)
  }
docs: // Wrap adds a transport middleware function that will give the caller
  // an opportunity to wrap the underlying http.RoundTripper prior to the
  // first API call being made. The provided function is invoked after any
  // existing transport wrappers are invoked.