stats doesn't work on cgroup v2

[Mossaka]

When I ran ctr task metric <task-name>, the container runtime returns a rpc error, indicating that the metric collecting method doesn't work on cgroup v2. I think I know the issue lies in the collect_metric method. Will have a PR to fix it.

Reproduce

➜  rust-extensions-fork git:(main) sudo ctr run --net-host --rm --runtime=io.containerd.runc.v2-rs docker.io/library/py-flask-app:latest pyflask
 * Serving Flask app 'app'
 * Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on all addresses (0.0.0.0)
 * Running on http://127.0.0.1:8000
 * Running on http://10.0.0.4:8000
Press CTRL+C to quit
ctr: rpc error: code = Unknown desc = ttrpc: closed: unknown

➜  runwasi git:(issue64) ✗ sudo ctr task metric pyflask         
ctr: not found

[Mossaka]

I verified that stats() does work on cgroup v1 Linux

➜  ~ sudo ctr task metric nginx                                                                          
ID       TIMESTAMP                              
nginx    seconds:1690942917  nanos:205307933    

METRIC                   VALUE       
memory.usage_in_bytes    3174400     
memory.limit_in_bytes    0           
memory.stat.cache        0           
cpuacct.usage            37091300    
cpuacct.usage_percpu     []