code: func cacheExpiry(c context.Context) time.Duration {
  	rnd := time.Duration(mathrand.Int63n(c, int64(time.Minute)))
  	return 4*time.Minute + rnd
  }
docs: // cacheExpiry returns a random duration from [4 min, 5 min).
  //
  // It's used to define when to refresh in-memory Queryable cache. We randomize
  // it to desynchronize cache updates of different Policy instances.