K8s: enrich mentrics

[kaitlynmichael]

Note

Low Risk
Documentation-only changes with no runtime or configuration behavior in this repo; remaining risk is publishing unverified allowlist rules or Prometheus guidance noted in inline reviewer comments.

Overview
Adds a new guide Enrich database metrics with tags that explains how REC spec.metricsConfig (allowlisted tag keys) and REDB/REAADB tag fields combine so selected database tags can surface in Prometheus-style monitoring.

The page documents the full metricsConfig field table, REDB spec.tags and REAADB spec.globalConfigurations.tags examples, operator-managed tag merge/deletion via status.managedTags, reserved keys, limitations, and a stub View tagged metrics in Prometheus section with reviewer questions on endpoint and PromQL.

Cross-links were added from database management, Active-Active database creation, and the Prometheus operator export article so readers can find the new topic from existing workflows.

^{Reviewed by Cursor Bugbot for commit 6a5e94c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

DOC-6702

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6702/operate/kubernetes/active-active/create-reaadb/
https://redis.io/docs/staging/DOC-6702/operate/kubernetes/re-clusters/connect-prometheus-operator/
https://redis.io/docs/staging/DOC-6702/operate/kubernetes/re-databases/db-controller/
https://redis.io/docs/staging/DOC-6702/operate/kubernetes/re-databases/enrich-metrics-with-tags/

[dwdougherty]

Language LGTM.