feat: add observability-metrics-gcp-cloudmonitoring module

[JanakaSandaruwan]

Purpose

Adds a new OpenChoreo observability metrics adapter backed by GCP Cloud
Monitoring. It serves per-component CPU/memory time series from the GKE
system metrics Google's built-in agent publishes for every GKE cluster, and
manages alert rules as Cloud Monitoring alert policies — no data-plane
collector required.

Approach

• Resource metrics query (POST /api/v1/metrics/query): six parallel
  ListTimeSeries calls against k8s_container, scoped by the OpenChoreo
  component/project/environment-uid metadata labels (UID-only, matching the
  Prometheus and Azure Monitor siblings). HTTP RED metrics return an empty
  series; runtime topology returns 501.
• Alert rule management (POST/GET/PUT/DELETE /api/v1alpha1/alerts/rules):
  maps each rule to a Cloud Monitoring AlertPolicy as a usage÷limit ratio
  MetricThreshold (threshold is a percentage of the pod's limit), using GCP's
  native denominator filter. Dedup/lookup via policy user_labels. All six
  operators supported natively.
• Webhook forwarding (POST /api/v1alpha1/alerts/webhook): parses the
  Cloud Monitoring incident payload and forwards firing alerts to the
  Observer's internal endpoint. The public webhook path is guarded by a
  shared-secret check (Basic-auth password / header / query token).
• Helm chart (helm/): Deployment + Service + ServiceAccount + ConfigMap,
  plus optional webhook Secret, Gateway HTTPRoute, and NetworkPolicy. Alerting
  is enabled only when a notification channel is configured; metrics-only
  installs need just roles/monitoring.viewer.
• README aligned with the sibling observability modules (Architecture,
  Prerequisites, IAM roles, Helm install, alerting, troubleshooting,
  configuration reference).
• Authentication via Application Default Credentials / GKE Workload Identity.
  Boot-time fail-fast ping of Cloud Monitoring (and notification-channel verify
  when alerting is on).

Related Issues

N/A

Checklist

• Tests added or updated (unit, integration, etc.)
• Samples updated (if applicable)

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.59546% with 314 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
observability-metrics-gcp-cloudmonitoring/main.go	0.00%	90 Missing ⚠️
...cloudmonitoring/internal/cloudmonitoring/alerts.go	45.27%	73 Missing and 8 partials ⚠️
...ity-metrics-gcp-cloudmonitoring/internal/server.go	0.00%	42 Missing ⚠️
...cs-gcp-cloudmonitoring/internal/handlers_alerts.go	72.34%	28 Missing and 11 partials ⚠️
...cloudmonitoring/internal/cloudmonitoring/client.go	72.60%	20 Missing ⚠️
...ity-metrics-gcp-cloudmonitoring/internal/config.go	66.66%	13 Missing and 3 partials ⚠️
...oring/internal/cloudmonitoring/alert_translator.go	93.38%	5 Missing and 4 partials ⚠️
...s-gcp-cloudmonitoring/internal/handlers_metrics.go	91.50%	7 Missing and 2 partials ⚠️
...cs-gcp-cloudmonitoring/internal/observer/client.go	76.92%	3 Missing and 3 partials ⚠️
...ics-gcp-cloudmonitoring/internal/webhook/parser.go	95.45%	1 Missing and 1 partial ⚠️

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