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>

Vagrantfile

@@ -228,6 +228,7 @@ EOF
         set -eux -o pipefail
         rm -rf /var/lib/containerd-test /run/containerd-test
         cd ${GOPATH}/src/github.com/containerd/containerd
+        go test -v -count=1 -race ./metrics/cgroups
         make integration EXTRA_TESTFLAGS="-timeout 15m -no-criu -test.v" TEST_RUNTIME=io.containerd.runc.v2 RUNC_FLAVOR=$RUNC_FLAVOR
     SHELL
   end