Capture number of IO operations performed with cgroup v2 hierarchy

[iyashu]

In cgroup v1, GetStats api captures the number of IO operations performed under IoServicedRecursive field of BlkioStats struct. But the same stat is missing with cgroup v2 implementation. As you can see here, we never populate the IoServicedRecursive field for cgroup v2. This resulted in misleading zero valued metrics in kubelet and cadvisor for cgroup v2. As per the cgroup v2 documentation, io.stat file has rios and wios fields denoting the number of read and write operations performed on a device. Let me know if I can add these stats for cgroup v2 as well?

Original cAdvisor Issue - google/cadvisor#2881

[iyashu]

cc/ @dims @mrunalp @AkihiroSuda

[dims]

@iyashu +1 to ensure the v2 impl faithfully reflects what we get in cgroup v1 impl.

[cyphar]

I will write a PR for this. Hopefully this will be the last PR for 1.0.0 (I was just getting ready to do the release).

FYI: You don't have to open an issue for fairly minor PRs -- if it's a fairly obvious fix for a regression with Kubernetes, you can just open a PR and we'll review it as-is (in fact we usually do most reviews and design discussions through PRs unless it's really complicated).