type RoundTripperFunc func(*http.Request) (*http.Response, error)

func (f RoundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
  return f(r)
}

func addAuthRoundTripper(apiKey string) http.RoundTripper {
  return http.RoundTripperFunc(func(r *http.Request) (*http.Response, error) {
    r.SetBasicAuth("", apiKey) // Not super secure, just using as an example
    return http.DefaultTransport.RoundTrip(r)
  })
}

type authRoundTripper struct {
  apiKey string
}

func (a *authRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
  r.SetBasicAuth("", apiKey)
  return http.DefaultTransport.RoundTrip(r)
}