Add a built-in cert-manager integration and dashboard

[hriszc]

Closes #6023

Summary

• add a new built-in cert_manager integration with setup docs, metadata, and icon assets
• ship a cert-manager overview dashboard covering certificate inventory, issuer readiness, challenge state, and ACME request rate
• include both standard and _dot dashboard definitions so the integration works with either metrics mode

Validation

• jq empty pkg/query-service/app/integrations/builtin_integrations/cert_manager/integration.json pkg/query-service/app/integrations/builtin_integrations/cert_manager/assets/dashboards/overview.json pkg/query-service/app/integrations/builtin_integrations/cert_manager/assets/dashboards/overview_dot.json
• git diff --check

Notes

• this environment does not have go, so I could not run the Go test/build pipeline here

---

Note

Low Risk
Asset-only change (JSON/Markdown/SVG) that doesn’t touch runtime logic; main risk is dashboard/query correctness and expected metric/label availability.

Overview
Adds a new built-in cert_manager integration (metadata, icon, and overview docs) under builtin_integrations/.

Ships a cert-manager overview dashboard (plus an _dot variant) with filters for namespace/issuer/certificate and panels for certificate inventory, readiness, challenge status, and ACME request rate, along with collector scrape/configuration instructions.

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[cursor]

Dot dashboard uses underscore metric names instead of dots

High Severity

The overview_dot.json file is an exact copy of overview.json — all metric names and variable queries still use underscore notation (e.g., certmanager_certificate_expiration_timestamp_seconds) instead of dot notation (e.g., certmanager.certificate.expiration.timestamp.seconds). When dot metrics mode is enabled, builtin.go swaps in the _dot.json dashboard, but since it contains the wrong metric names, every panel and variable query will match zero data and the dashboard will appear empty.