New metrics and logging for golden-base VM reuse and provision work

Added metrics to observe golden-base reuse: tart_kubelet_golden_base_reused_total{pool} tracks warm clonefile hits, and tart_kubelet_vm_provision_work_seconds{pool,path=warm|cold} isolates the on-host pull/clone time from queue wait. An error log also surfaces when a tart get falls through to a cold re-pull for reasons other than a missing image, making it easier to diagnose why the golden warm path is missed.