support cgroup2

* only shim v2 runc v2 ("io.containerd.runc.v2") is supported
* only PID metrics is implemented. Others should be implemented in separate PRs.
* lots of code duplication in v1 metrics and v2 metrics. Dedupe should be separate PR.

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

metrics/types/v2/types.go

@@ -0,0 +1,34 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package v2
+
+import (
+	v2 "github.com/containerd/cgroups/v2/stats"
+)
+
+type (
+	// Metrics alias
+	Metrics = v2.Metrics
+	// MemoryStat alias
+	MemoryStat = v2.MemoryStat
+	// CPUStat alias
+	CPUStat = v2.CPUStat
+	// PidsStat alias
+	PidsStat = v2.PidsStat
+)