ctr: fix potential panic in metric
ctr: fix potential panic in metric

Signed-off-by: Ace-Tang <a967bc6ab01ace27203ca6ab9fd77fbd3b598184@126.com>

estesp/containerd,containerd/containerd,mikebrow/containerd,thaJeztah/containerd,containerd/containerd,dmcgowan/containerd,dmcgowan/containerd,thaJeztah/containerd,estesp/containerd,miaoyq/containerd,crosbymichael/containerd,crosbymichael/containerd,miaoyq/containerd,vdemeester/containerd,vdemeester/containerd,mikebrow/containerd