Add write buffer to  AiUserMetrics model

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179244



Merged-by: Jan Provaznik <jprovaznik@gitlab.com>
Approved-by: Dillon Wheeler <dwheeler@gitlab.com>
Approved-by: Jan Provaznik <jprovaznik@gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Reviewed-by: Jan Provaznik <jprovaznik@gitlab.com>
Reviewed-by: Pavel Shutsin <pshutsin@gitlab.com>
Reviewed-by: Dillon Wheeler <dwheeler@gitlab.com>
Reviewed-by: Bishwa Hang Rai <bhrai@gitlab.com>
Co-authored-by: Pavel Shutsin <pshutsin@gitlab.com>