✨ MachinePool annotation for externally managed autoscaler

[jackfrancis]

What this PR does / why we need it:

This PR proposes a standard MachinePool annotation that can be used to indicate that replica enforcement is under the control of an external (in the sense that it is not native to cluster-api) autoscaler solution. This is currently useful in Managed Kubernetes scenarios, but could also be potentially used by domain/provider-specific solutions that operate against native infra metrics to determine autoscaling criteria and don't have a way to communicate with cluster-api.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[jackfrancis]

cc @CecileRobertMichon @mboersma

[CecileRobertMichon]

FYI @dthorsen

This is an alternative solution to #5685

[CecileRobertMichon]

/lgtm

[CecileRobertMichon]

/assign @enxebre

[sbueringer]

Last nits from my side, otherwise lgtm

[sbueringer]

/lgtm

/assign @vincepri
/assign @CecileRobertMichon

[CecileRobertMichon]

/lgtm

[jackfrancis]

@vincepri do we want to punt on including this in 1.3? (not much time left)

[sbueringer]

@vincepri do we want to punt on including this in 1.3? (not much time left)

I'm in general in favor of handling things pragmatically. Especially as in this case:

• we discussed the topic already last week in office hours (and there were no objecttions)
• we had 2 lgtm's of maintainers (both before the RC)

But given:

• Week 15-16: RC
  • There is a feature freeze on the release branch
    • Only cherry-picks for documentation, bug fixes, test fixes and test additions are allowed

I would like to hear opinions from other maintainers
(cc @CecileRobertMichon @enxebre @fabriziopandini @vincepri)

P.S. To be clear, if we want to get this into v1.3 we have to merge this PR first and then cherry-pick it into release-1.3.

[CecileRobertMichon]

I could be convinced either way 🤷‍♀️

[AverageMarcus]

Having this included in the 1.3 release would be useful for providers as work could start right away on adding this support - CAPZ update to use the new annotation, CAPA to merge the related open PR, etc.

[vincepri]

I'm fine including this in 1.3

/lgtm

[sbueringer]

Okay let's do it :)
(given reasons mentioned in: #7107 (comment))

/approve

/cherry-pick release-1.3

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.3 in a new PR and assign it to you.

In response to this:

Okay let's do it :)

/approve

/cherry-pick release-1.3

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[k8s-ci-robot]

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #7560

In response to this:

Okay let's do it :)
(given reasons mentioned in: #7107 (comment))

/approve

/cherry-pick release-1.3

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.