Move metrics to core/metrics

Signed-off-by: Derek McGowan <derek@mcg.dev>

Vagrantfile

@@ -257,7 +257,7 @@ EOF
         set -eux -o pipefail
         rm -rf /var/lib/containerd-test /run/containerd-test
         cd ${GOPATH}/src/github.com/containerd/containerd
-        go test -v -count=1 -race ./metrics/cgroups
+        go test -v -count=1 -race ./core/metrics/cgroups
         make integration EXTRA_TESTFLAGS="-timeout 15m -no-criu -test.v" TEST_RUNTIME=io.containerd.runc.v2 RUNC_FLAVOR=$RUNC_FLAVOR
     SHELL
   end