code: func setCacheMeta(resp http.ResponseWriter, m *cache.ResultMeta) {
  	if m == nil {
  		return
  	}
  	str := "MISS"
  	if m.Hit {
  		str = "HIT"
  	}
  	resp.Header().Set("X-Cache", str)
  	if m.Hit {
  		resp.Header().Set("Age", fmt.Sprintf("%.0f", m.Age.Seconds()))
  	}
  }
docs: // setCacheMeta sets http response headers to indicate cache status.