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/memory.go

@@ -586,4 +586,20 @@ var memoryMetrics = []*metric{
 			}
 		},
 	},
+	{
+		name: "memory_oom",
+		help: "The number of times a container has received an oom event",
+		unit: metrics.Total,
+		vt:   prometheus.GaugeValue,
+		getValues: func(stats *v2.Metrics) []value {
+			if stats.MemoryEvents == nil {
+				return nil
+			}
+			return []value{
+				{
+					v: float64(stats.MemoryEvents.Oom),
+				},
+			}
+		},
+	},
 }