code: func FilterHandler(fn func(r *Record) bool, h Handler) Handler {
  	return FuncHandler(func(r *Record) error {
  		if fn(r) {
  			return h.Log(r)
  		}
  		return nil
  	})
  }
docs: // FilterHandler returns a Handler that only writes records to the
  // wrapped Handler if the given function evaluates true. For example,
  // to only log records where the 'err' key is not nil:
  //
  //    logger.SetHandler(FilterHandler(func(r *Record) bool {
  //        for i := 0; i < len(r.Ctx); i += 2 {
  //            if r.Ctx[i] == "err" {
  //                return r.Ctx[i+1] != nil
  //            }
  //        }
  //        return false
  //    }, h))
  //