req2 := req.Clone(req.Context())
	req2.Header.Set("User-Agent", u.ua)
	return u.rt.RoundTrip(req2)
}

// WithUserAgent wraps an existing transport with custom User-Agent
func WithUserAgent(rt http.RoundTripper, getUA func() string) http.RoundTripper {
	return &uaTransport{
		ua: getUA(),
		rt: rt,
	}

	if err != nil {
		return nil, err
	}

	client, err := storage.NewClient(context.Background(),
		option.WithCredentialsJSON(credsJSON),
		option.WithScopes(storage.ScopeReadWrite),
		option.WithUserAgent(fmt.Sprintf("gcs-tier-%s", tier)+SlashSeparator+ReleaseTag),
	)
	if err != nil {
		return nil, err
	}
	return &warmBackendGCS{client, conf.Bucket, conf.Prefix, conf.StorageClass}, nil
}