PO to GMP Migration Tool: ServiceMonitor Spec Migration

[karthunni]

• ServiceMonitor Conversion & Registration: Implements ServiceMonitorConverter in pkg/migrate/servicemonitor.go and registers it in the CLI entrypoint (cmd/gmp-migrate/main.go).
• Conflict-Aware Resource Splitting: Resolves backing Kubernetes Service resources from the ResourceCache matching spec.selector. Groups Services by configuration compatibility (selectors, port mappings, labels) and automatically splits conflicting Services into distinct, uniquely-named PodMonitoring resources.
• Namespace & Scoping Resolution: Evaluates spec.namespaceSelector to generate either namespace-scoped PodMonitoring resources (cloned across all targeted namespaces) or cluster-scoped ClusterPodMonitoring resources when any: true is specified.
• Endpoint & Label Translation: Translates endpoint ports, paths, schemes, intervals, timeouts, and authentication configurations. Merges Service target labels (spec.targetLabels), Pod target labels (spec.podTargetLabels), spec.jobLabel, and pre-scrape relabeling rules into GMP scrape endpoints and label selectors.
• Namespace-Scoped Secret Generation: Ensures generated authentication secrets (bearer tokens, basic auth, TLS certificates) are properly scoped and cloned per target namespace (pkg/migrate/helpers.go).
• Testing: Adds table-driven unit tests in pkg/migrate/servicemonitor_test.go covering multi-service conversion, conflict splitting, cluster-scoped monitoring, multi-namespace cloning, and secret generation.

[gemini-code-assist]

Code Review

This pull request introduces a ServiceMonitorConverter to translate Prometheus Operator ServiceMonitor resources into Google Managed Prometheus (GMP) PodMonitoring or ClusterPodMonitoring resources, sharing common spec logic with the existing PodMonitorConverter. The review feedback highlights several critical resilience issues where non-fatal configuration discrepancies—such as missing backing Services, Services without selectors, or unresolvable ports—incorrectly trigger fatal migration failures instead of logging warnings and using placeholders. Additionally, the reviewer noted that static target labels mapped from Services must be checked against GMP's protected labels to prevent validation failures.