feat: implement agent_sandbox_creation_latency_ms metric

[chw120]

This PR introduces the agent_sandbox_creation_latency_ms metric to track the time it takes from a Sandbox's creation to when its associated Pod reaches a Ready state. This provides better visibility into the performance of sandbox provisioning across different launch types and templates.

Key Changes

• New Prometheus Histogram: Defined SandboxCreationLatency as a histogram vector in internal/metrics/metrics.go.

  • Buckets: Configured with ranges from 50ms up to 4 minutes (50, 100, 250, 500, 1000, 2500, 5000, 10000, 30000, 60000, 120000, 240000).
  • Labels: Includes launch_type ("warm", "cold", "unknown") and sandbox_template to allow granular analysis.

  Note: I did not add "status" defined in the original requirement, since it would be always success.

• Controller Integration: Updated the SandboxClaimReconciler in extensions/controllers/sandboxclaim_controller.go to calculate and record this latency during the reconciliation process. It specifically measures the duration between sandbox.CreationTimestamp and the Pod's LastTransitionTime for the Ready condition.

• Helper Method: Added RecordSandboxCreationLatency to the internal metrics package for clean invocation from controllers.

Testing

• Unit Tests: Added TestSandboxLatencyRecording in internal/metrics/metrics_test.go to ensure that observations are correctly recorded and grouped by their respective launch type labels.

Working on #245

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[linux-foundation-easycla]

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• ✅ login: chw120 / name: Chenyi Wang (5658897, 5cea8f9, 6f337e5, a1e578b, c7c71ae, d434c9f)

[k8s-ci-robot]

Hi @chw120. Thanks for your PR.

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[chw120]

/easycla

[chw120]

/easycla

[yongruilin]

@chw120 I don't think Jules signed the CLA . You might need to configured the Jules to use "User Only" mode to get rid of the co-authored.
/ok-to-test

[aditya-shantanu]

/assign igooch

[igooch]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chw120, igooch

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