feat: create dashboard to monitor CI partitions

Related to gitlab-org/gitlab#461165, it creates a dedicated dashboard to monitor the database stats when changing CI partitions.

marius@rocket-sled ~/W/r/dashboards (mb-create-ci-partitioning-dashboard)> ./test-dashboard.sh ./ci-partitioning/main.dashboard.jsonnet
Installed https://dashboards.gitlab.net/dashboard/snapshot/XKiF7DeY3VPxfZSfTq4LrMJsxklOSU8J?var-environment=gprd&orgId=1&var-PROMETHEUS_DS=mimir-gitlab-gprd&var-stage=main - ci-partitioning: CI data partitions tracking

Installed https://dashboards.gitlab.net/dashboard/snapshot/XKiF7DeY3VPxfZSfTq4LrMJsxklOSU8J?var-environment=gprd&orgId=1&var-PROMETHEUS_DS=mimir-gitlab-gprd&var-stage=main - ci-partitioning: CI data partitions tracking

If you get Unauthorized/page not found errors, you might need to login again https://dashboards.gitlab.net/login