Change ClaimStartupLatency to use an annotation for creation timestamp

[igooch]

Modifies the ClaimStartupLatency metric to use a new agents.x-k8s.io/webhook-first-seen-at annotation for the start time, providing millisecond granularity. This replaces the use of the Kubernetes creation timestamp, which only has second precision, and resulted in inaccurate metrics.

Note that this metric is distinct from ControllerStartupLatency which only includes latency after the SandboxClaim has been seen by the agent-sandbox controller.

The actual webhook to add this annotation must be provided by the user or cloud provider. This PR only consumes the annotation.

Note for reviewer:

This PR is broken into two separate commits. The first commit has the functional changes. The second commit is refactoring only.

#245

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[aditya-shantanu]

@igooch -> Why does it have to be a webhook added timestamp ?
I can imagine that the client SDK is setting this instead ? (if the client so chooses ?)

• my point being - let's rename the annotation to be a bit more generic ?