feat: Add grafana2 Helm chart for staging GCP cluster with dashboards as code

[tomadowley]

This patch introduces the grafana2 Helm chart to deploy Grafana on the staging GCP cluster with dashboards-as-code support.

What’s included:

• grafana2 Helm chart that wraps the Grafana Community chart (grafana 12.3.0) to stay on a maintained chart line
• Dashboards as code: dashboards/*.json are provisioned as ConfigMaps via dashboard-configmaps.yaml and loaded by Grafana through the dashboard sidecar
• Data sources as code: Google Cloud Monitoring (stackdriver) and Google Cloud Logging configured via a datasources-configmap.yaml and Grafana sidecar
• Secrets integration: expects a 1Password secret named cluster-staging-grafana in the target namespace containing DB_URL, admin-user, and admin-password; Grafana admin credentials are read from this secret
• Staging values: grafana2/values-staging.yaml and grafana2/values.yaml define project, replicas, image, RBAC, sidecar provisioning, and Google Cloud data sources
• Install guidance: after dependency update, install with Helm in the observability namespace using the staging values; example in README

Files added/updated:

• grafana2/Chart.yaml, grafana2/Chart.lock, grafana2/README.md, grafana2/templates/dashboard-configmaps.yaml, grafana2/templates/datasources-configmap.yaml, grafana2/templates/_helpers.tpl, grafana2/charts/.gitignore, grafana2/dashboards/gke-managed-prometheus-overview.json, grafana2/values.yaml, grafana2/values-staging.yaml, grafana2/values.yaml
• grafana2/dashboards and grafana2/values-staging.yaml provide the configuration for dashboards and GCP data sources in staging

How this solves the task:

• Enables Helm-based deployment of Grafana to the staging GCP cluster with dashboards and data sources provisioned as code, ensuring consistent environments and easier rollout between stages.
• Keeps Grafana on a maintained chart line by wrapping grafana-community/grafana, while allowing dashboards and data sources to be managed as code and provisioned automatically via sidecars.
• Uses a single secret source (1Password) for DB credentials and admin user/password, keeping sensitive data out of charts.