// CountingTransport will count requests.
type CountingTransport struct {
  N    int64              // number of requests passing this transport
  next http.RoundTripper  // next round-tripper or http.DefaultTransport if nil
}
// RoundTrip implements a transport that will count requests.
func (tr *CountingTransport) RoundTrip(r *http.Request) (*http.Response, error) {
  atomic. AddInt64(&tr.N, 1)
  if tr.next != nil {
    return tr.next.RoundTrip(r)
  }
  return http.DefaultTransport.RoundTrip(r)
}
...
myHttpClient := &http.Client{Transport: &CountingTransport{}}
client, err := elastic.NewClient(elastic.SetHttpClient(myHttpClient))

// BasicAuthTransport 
type BasicAuthTransport struct {
  username string
  password string
}
func (tr *BasicAuthTransport) RoundTrip(r *http.Request) (*http.Response, error) {
  r.SetBasicAuth(tr.username, tr.password)
  return http.DefaultTransport.RoundTrip(r)
}
...
httpClient = &http.Client{
  Transport: &BasicAuthTransport{
    username:  "me",
    password:  "secret",
  },
}
client, err := elastic.NewClient(elastic.SetHttpClient(httpClient))