cgroup: add podruntime.slice to recognized systemd slices

[Alegrowin]

Summary

Adds podruntime.slice to the systemd slice regex and prefix switch in containerByCgroup(), following the same pattern as #264 (which added kube.slice and azure.slice).

Without this fix, clusters using kubeReservedCgroup: /podruntime.slice log hundreds of warnings per scrape cycle:

W0303 - registry.go:334] failed to read proc cgroup: unknown container: /podruntime.slice/kubelet.service

Context

/podruntime.slice is the upstream Kubernetes recommended name for kube-reserved-cgroup. It is used by OVHcloud MKS and other distributions that follow the Node Allocatable design proposal. Kubelet and containerd are placed under this slice via systemd Slice=podruntime.slice.

Changes

• cgroup.go: Add podruntime to systemSliceIdRegexp and add podruntime.slice to the prefix case
• cgroup_test.go: Add test cases for /podruntime.slice/containerd.service and /podruntime.slice/kubelet.service

[def]

@Alegrowin thank you!