Establish Envoy Gateway observability and alerting

Summary

Set up comprehensive observability, monitoring, and alerting for Envoy Gateway deployments in pre and staging environments.

Acceptance Criteria

• Baseline metrics collected (latency, error rates, connection health)
• Dashboards created for operational visibility
• Alerts configured for critical issues
• Observability approach documented
• Team trained on monitoring and troubleshooting

Related

• Parent Epic: [Phase 1] Multi-platform Gateway/Proxy: GitLab.com Pilot in Non-Production Environments (#1644 (closed))
• Coordinate with: groupoperate team on Envoy Gateway observability offerings