Description
cgroup2: metrics fails after running several times

Steps to reproduce the issue:

1. Set up cgroup v2 host (Ubuntu 19.10)
2. Run sudo ctr run -t --rm --runc-binary=crun docker.io/library/alpine:latest foo
3. Run sudo ctr t metrics foo several times for a few minutes

Describe the results you received:
It succeeds for a while, but eventually it begin to fail with collecting metrics for foo error="cgroups: parsing file with invalid format failed: unknown"

Describe the results you expected:
Should succeed every time.

Output of containerd --version:

containerd github.com/containerd/containerd v1.3.0-247-g9f193c41 9f193c41d74e815480c2f69964411f9b87599902