fix(function-autoscaler): re-evaluate metric source after first LLM request

[borao]

Describe the bug

In autoscaler 1.21.0, a running LLM function with no prior requests has no worker or per-function gateway series. The autoscaler selects ControlPlane and caches it for one hour, so later LLM traffic remains invisible and the function cannot scale correctly.

Steps or code to reproduce bug

1. Deploy a new LLM function with minInstances: 1.
2. Wait 2–3 minutes without sending requests.
3. Send sustained LLM traffic.
4. Observe that the autoscaler continues using ControlPlane metrics and reports 0% utilization.

Expected behavior

Absence-based fallback is re-evaluated after gateway metrics appear, and missing utilization data cannot trigger scale-to-zero.

Additional context

• Parent tracking issue: Integrate function autoscaler into the self-managed stack #15
• Add coverage for traffic beginning after initial metric-source selection.

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