VSD - Group merge request throughput should include subgroups

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143377



Merged-by: Savas Vedova <svedova@gitlab.com>
Approved-by: Sharmad Nachnolkar <snachnolkar@gitlab.com>
Approved-by: Savas Vedova <svedova@gitlab.com>
Co-authored-by: Alex Pennells <apennells@gitlab.com>