Linux runner jobs now populate the Metrics tab with in-VM machine metrics

The Metrics tab on a runner job now shows real CPU, memory, network, and disk usage data for jobs that run on the Linux runner image, instead of staying empty. The Linux runner now ships a dedicated metrics sidecar inside the microVM that samples the job’s resource use for its full duration and reports it back to the server. Sampling is best-effort, so a bad sample never affects the build.