feat(apigee): Add Apigee API Health & Availability dashboard

[Baazigar007]

Description

This Pull Request adds a new public dashboard template for Apigee API Health & Availability to the public sample library. This dashboard allows users to monitor the direct impact on API availability, latency, and success rates at a glance, using standard re-exported metrics from Apigee.

Changes

• Dashboard JSON (dashboards/apigee/health-availability.json): Configures the dashboard widgets with high-fidelity V2 metrics including:
  • Traffic (request rate scorecard)
  • Latency (P95 scorecard)
  • 5xx Error Rate scorecard
  • Request rate breakdown by status code
  • 5xx and 4xx error rates breakdown by proxy
• Metadata (dashboards/apigee/metadata.yaml): Registers the new health-availability dashboard under the Apigee category.
• Documentation (dashboards/apigee/README.md): Adds a table entry and a summary description of the new dashboard file.
• Screenshot (dashboards/apigee/health-availability.png): Adds a preview of the rendered dashboard layout.

Verification

• Visually verified the layout matches golden monitoring signals (Traffic, Errors, and Latency) - https://sre.google/sre-book/monitoring-distributed-systems/#xref_monitoring_golden-signals.
• Verified the JSON file structure using the local import script to a GCP test project:
  scripts/dashboard/dashboard.sh import <project-id> dashboards/apigee/health-availability.json
• The dashboard was successfully rendered on the GCP console without visual errors or warnings.

[Baazigar007]

Removed blank fields.

[SamiSousa]

LGTM