Volume stats support

[zuzzas]

What this PR does / why we need it:

Support NodeGetVolumeStats CSI call.

gRPC error problem
There is one problem that I haven't tackled. How do I verify that some fs path corresponds to volume ID without something similar to PublishContext?

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[codenrhoden]

Hi @zuzzas. Thank you for this PR! I have a couple different types of feedback for you...

There is one problem that I haven't tackled. How do I verify that some fs path corresponds to volume ID without something similar to PublishContext?

I see your point, and I had similar issues with NodeUnstageVolume, and I raised the issue against the spec here. I did get an answer there, which is that the SP needs to store that PublishContext info itself, somehow, because the CO will not persist that information. So the easiest solution is probably to write the info to disk during the NodeStageVolume call... But I never did that so far, because...

This specific driver is about to get a fully new implementation by the VMware Business Unit responsible for Storage. This will be a much more fully-featured driver, suitable for production workloads, and fully supported. This is a reason why this driver has remained in alpha state. It is not intended for this current implementation to live on once the new one is available, and I believe we will see the new one within the next week or two.

Once that becomes available, it may very well be the case that it could use an implementation for NodeGetVolumeStats, I honestly haven't looked to see if that's been done.

[zuzzas]

and I believe we will see the new one within the next week or two.

Thanks for the quick response. We'll wait for it, then. And, perhaps, find ways to collaborate on the next implementation.