code: func WithTags(ctx context.Context, tags []string) context.Context {
  	stats, ok := FromContext(ctx)
  	if !ok {
  		return ctx
  	}
  	return WithStats(ctx, &taggedStats{tags, stats})
  }
docs: // WithTags will return a context.Context where all metrics recorded with the
  // embedded Stats implementation will include the given stats.