metrics/cgroups: fix deadlock issue in Add during Collect

The Collector.Collect will be the field ns'Collect's callback, which be
invoked periodically with internal lock. And Collector.Add also runs
with ns.Lock in Collector.Lock, which is easy to cause deadlock.

Goroutine X:

	ns.Collect
	  ns.Lock
	    Collector.Collect
	      Collector.RLock

Goroutine Y:

	Collector.Add
	  Collector.Lock
	    ns.Lock

We should use ns.Lock without Collector.Lock in Add.

Fix: #6772

Signed-off-by: Wei Fu <fuweid89@gmail.com>

metrics/cgroups/common/type.go

@@ -0,0 +1,33 @@
+//go:build linux
+// +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 common
+
+import (
+	"context"
+
+	"github.com/gogo/protobuf/types"
+)
+
+// Statable type that returns cgroup metrics
+type Statable interface {
+	ID() string
+	Namespace() string
+	Stats(context.Context) (*types.Any, error)
+}