(feat) extend health checks by adding metric support

[gianlucam76]

ValidateHealth now supports metric-based checks via a new MetricSource field. The addon-controller (push mode) or the sveltos-applier agent running inside the managed cluster (pull mode) queries a Prometheus-compatible endpoint, collects named scalar values, and exposes them to a Lua script through a global metrics table.

In push mode the MetricSource URL must be reachable from the management cluster. In pull mode the agent running inside the managed cluster reaches the metric service directly via its in-cluster DNS name.

Example:

  validateHealths:
  - name: error-rate-low
    featureID: Helm
    metricSource:
      url: http://prometheus-server.monitoring.svc:9090
    metricQueries:
    - name: errorRate
      query: >-
        sum(rate(http_requests_errors_total{namespace="my-app"}[5m]))
        /
        sum(rate(http_requests_total{namespace="my-app"}[5m]))
    script: |
      function evaluate()
        if metrics["errorRate"] > 0.05 then
          return {healthy=false, message="error rate above 5%: " .. metrics["errorRate"]}
        end
        return {healthy=true, message=""}
      end

ValidateHealths are skipped during ClusterSummary deletion so that health checks do not block resource teardown when the managed workloads are already gone.