Add oldest instance age metric by state

[sjmiller609]

Summary

• add hypeman_instances_oldest_in_state_seconds as an observable gauge grouped by state and hypervisor
• compute the oldest current instance age per state alongside the existing hypeman_instances_total callback
• add a focused unit test covering stopped and standby instances with different ages

Testing

• go test ./lib/instances -run 'TestSnapshotCompressionMetrics_RecordAndObserve|TestInstanceOldestInStateMetric_ObserveOldestAgePerState'

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Note

Low Risk
Low risk: adds a read-only OpenTelemetry gauge and a unit test; behavior change is limited to metrics emission and clamps invalid/negative ages to 0.

Overview
Adds a new observable gauge, hypeman_instances_oldest_in_state_seconds, labeled by state and hypervisor, computed in the same callback as hypeman_instances_total by tracking the maximum age since CreatedAt (with zero/negative ages ignored or clamped).

Extends metrics_test.go with a focused test that creates multiple instances across stopped/standby states and verifies both the per-state counts and the reported oldest age values.

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