fix(cloudprovider/exteranlgrpc): properly handle unimplemented methods

[dbonfigli]

What type of PR is this?

/kind bug
/kind feature

What this PR does / why we need it:

Some CloudProvider and nodeGroup methods are optional: if a cloud provider does not implemented an optional method, it should return the cloudprovider.ErrNotImplemented error, this error is expected and the CA gracefully handles it.

Currently there is no way to pass this specific error from a grpc based cloud provider: The protobuf definition incorrectly states that an rpc is optional or a grpc error will result in using defaults, but this is not correct at the moment: in case of grpc errors, the client passes a generic error that is not correctly handled by the CA, so currently all rpc must be implemented.

This PR allows grpc cloud providers to not implement optional methods: a not implemented method should pass the grpc error code 12 Unimplemented, if so, the CA grpc client will transform this to the go cloudprovider.ErrNotImplemented error that is then correctly handled.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

externalgrpc cloud provider: grpc based cloud providers can now pass the grpc error code 12, Unimplemented, to signal they do not implement optional methods.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[dbonfigli]

keeping this on draft until #5936 is merged so to cleanly regenerate the code for the protobuf.

[dbonfigli]

after merging #5936, this is now ready to be reviewed, tagging @vadasambar as you expressed interest on this part of the code and i am the only one listed (informally, commented out) on the OWNERS file for this cloud provider.

[vadasambar]

nit: the wording feels a little ambiguous. Here's a suggestion:

Suggested change

	// Implementation optional: if so should return error code 12 Unimplemented
	// Implementation optional: if unimplemented return error code 12 (for `Unimplemented`)

[dbonfigli]

ok, suggestion accepted

[vadasambar]

Overall looks good. I will do another pass and approve it.

[vadasambar]

/assign vadasambar

[vadasambar]

/lgtm

[dbonfigli]

after the lgtm label , I am assigning this to @towca for the approval.

/assign @towca

[x13n]

/approve

@dbonfigli Did you think about becoming k8s org member so you can approve changes for externalgrpc?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dbonfigli, vadasambar, x13n

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• cluster-autoscaler/OWNERS [x13n]

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[dbonfigli]

@dbonfigli Did you think about becoming k8s org member so you can approve changes for externalgrpc?

If you are willing to sponsor me and we find another reviewer that wants to sponsor me I am willing to keep contributing to this project as an official k8s org member.

[x13n]

Happy to do so! If you don't have another sponsor you could ask on the SIG meeting.

[vadasambar]

If you are willing to sponsor me and we find another reviewer that wants to sponsor me I am willing to keep contributing to this project as an official k8s org member.

Added it to the agenda for Monday: https://docs.google.com/document/d/1RvhQAEIrVLHbyNnuaT99-6u9ZUMp7BfkPupT2LAZK7w/edit#bookmark=id.xtfu11692zyn

[elmiko]

i'm happy to help with sponsoring as well, i reviewed some of the original grpc work.