Add row counts to registry migration dashboard

This MR modifies the registry migration dashboard to add row count metrics for the most important database tables. These serve as good indicators for the migration Phase 1 progress.

Additionally, it renames the dashboard to Migration Detail for consistency with the remaining registry dashboards.

https://dashboards.gitlab.net/d/KbvjANv7z/registry-migration-detail-5?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gstg&var-stage=main&var-migration_path=All

Note: This dashboard is temporary and unmanaged, so it needs to be applied manually.

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14304.