feat(metis): fallback CNI to direct local IPAM when daemon is unavailable

[arvindbr8]

Summary

This PR implements socket connection retry logic and direct local IPAM fallback in the Metis CNI plugin when the background daemon process is unavailable or starting up.

• Socket Availability Check & Connection Retries:
  • Adds socket file availability verification (os.Stat) prior to gRPC client initialization in getGrpcClient to prevent lazy gRPC connection false-positives.
  • Adds 3 connection retries with 100ms backoff in prepare() to gracefully handle transient daemon restarts or startup timing.
• Direct Local IPAM Fallback:
  • If the daemon socket remains unavailable after retries, the CNI plugin falls back to opening the SQLite store directly at dbPath (/var/lib/cni/metis/metis.sqlite) and instantiating IPAMEngine in-process via directClientAdapter.
  • Operates without initializing network/UDS listeners, gRPC servers, or background sync tasks.
  • Built-in default behavior without extra configuration knobs.

Operational & Safety Details

• SQLite Write-Ahead Logging (WAL), _busy_timeout=5000ms, and _txlock=immediate ensure transaction isolation and lock safety when CNI and Daemon concurrently access SQLite.
• If local IP capacity is exhausted while operating in direct mode, IPAMEngine cleanly returns store.ErrNoAvailableIPs (since no daemon/monitor is present to trigger dynamic scale-up).

Test Plan

• Unit Tests:
  • go test ./... in metis/ (ok k8s.io/metis/pkg/cni, ok k8s.io/metis/pkg/daemon, ok k8s.io/metis/pkg/store).
• Integration Tests:
  • Executed metis/test/daemon_cni_race_test.go covering deterministic startup sequences:
    • TestCNI_SequenceA_CNIBeforeDaemon: CNI runs before daemon exists -> direct fallback -> daemon starts -> daemon handles subsequent requests cleanly.
    • TestCNI_SequenceB_SocketAppearsDuringRetry: Daemon socket creation delayed by 150ms -> CNI retries socket -> connects via gRPC on attempt 2.
    • TestCNI_SequenceC_DaemonAlreadyRunning: CNI connects immediately to running daemon.
• Verification:
  • make verify-up-to-date passed with 0 dirty changes.

[kubernetes-prow]

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[arvindbr8]

/assign @YifeiZhuang

[kubernetes-prow]

Hi @arvindbr8. Thanks for your PR.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: arvindbr8
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[YifeiZhuang]

/ok-to-test