What is the problem you're trying to solve

It would be helpful to see metrics around pulling an image

inspired by kubernetes/kubernetes#111259

Describe the solution you'd like

• number of currently pulling images - gauge
• time of pulling an image (maybe histogram) ?
• total errors related to pulling - counter ?

Additional context

I am not sure if this should be added in kubelet or containerd. I prefer to add the metrics in container runtime.

In kubelet, the image pulling may hang due to serialize-image-pulls being true by default.

Further information, #4211, and #6150 are related to the observability of image pulling.