Google Cloud Artifact Registry: add user agent metadata

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148079



Merged-by: Dzmitry Meshcharakou <12459192-dmeshcharakou@users.noreply.gitlab.com>
Approved-by: Pedro Pombeiro <noreply@pedro.pombei.ro>
Approved-by: Dzmitry Meshcharakou <12459192-dmeshcharakou@users.noreply.gitlab.com>
Reviewed-by: Pedro Pombeiro <noreply@pedro.pombei.ro>
Reviewed-by: Dzmitry Meshcharakou <12459192-dmeshcharakou@users.noreply.gitlab.com>
Co-authored-by: David Fernandez <dfernandez@gitlab.com>