code: func AddOriginFields(rw http.ResponseWriter, req *http.Request, next http.Handler, data *LogData) {
  	crw := &captureResponseWriter{rw: rw}
  	start := time.Now().UTC()
  
  	next.ServeHTTP(crw, req)
  
  	// use UTC to handle switchover of daylight saving correctly
  	data.Core[OriginDuration] = time.Now().UTC().Sub(start)
  	data.Core[OriginStatus] = crw.Status()
  	// make copy of headers so we can ensure there is no subsequent mutation during response processing
  	data.OriginResponse = make(http.Header)
  	utils.CopyHeaders(data.OriginResponse, crw.Header())
  	data.Core[OriginContentSize] = crw.Size()
  }
docs: // AddOriginFields add origin fields