cgroup2: implement containerd.events.TaskOOM event

How to test (from https://github.com/opencontainers/runc/pull/2352#issuecomment-620834524):
  (host)$ sudo swapoff -a
  (host)$ sudo ctr run -t --rm --memory-limit $((1024*1024*32)) docker.io/library/alpine:latest foo
  (container)$ sh -c 'VAR=$(seq 1 100000000)'

An event `/tasks/oom {"container_id":"foo"}` will be displayed in `ctr events`.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

metrics/cgroups/v2/cgroups.go

@@ -21,10 +21,8 @@ package v2
 import (
 	"context"
 
-	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/events"
 	"github.com/containerd/containerd/runtime"
-	"github.com/containerd/containerd/runtime/v1/linux"
 	metrics "github.com/docker/go-metrics"
 )
 
@@ -48,19 +46,6 @@ func (m *cgroupsMonitor) Monitor(c runtime.Task) error {
 	if err := m.collector.Add(c); err != nil {
 		return err
 	}
-	t, ok := c.(*linux.Task)
-	if !ok {
-		return nil
-	}
-	cg, err := t.Cgroup()
-	if err != nil {
-		if errdefs.IsNotFound(err) {
-			return nil
-		}
-		return err
-	}
-	// OOM handler is not implemented yet
-	_ = cg
 	return nil
 }