code: func Since(ctx context.Context, key interface{}) time.Duration {
  	if startedAt, ok := ctx.Value(key).(time.Time); ok {
  		return time.Since(startedAt)
  	}
  	return 0
  }
docs: // Since looks up key, which should be a time.Time, and returns the duration
  // since that time. If the key is not found, the value returned will be zero.
  // This is helpful when inferring metrics related to context execution times.