Not send to graylog

Not send to graylog if field name = **duration**
Like this:
```golang
logFields := map[string]interface{}{
				"method":       r.Method,
				"url":          r.URL.Path + "?" + r.URL.RawQuery,
				"duration":    fmt.Sprintf("%f seconds", time.Since(startTime).Seconds()),
				"datetime":     startTime,
			}
```