fix: public Builder compatibility with the BuilderInterface

[clamoriniere]

What this PR does / why we need it:

This pull request add the changes introduced by #1864 into the "public" Builder (pkg/builder/Builder).

To avoid having future similar issue, this PR also add a interface type check between the pkg/build/type/BuilderInterface Interface and the pkg/build/Builder struct. It should allow to catch at compilation time any discrepancy.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

It doesn't affect cardinality of KSM because it doesn't change any metrics generation logic.

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 #

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[CatherineF-dev]

/lgtm

[clamoriniere]

Hi @CatherineF-dev

thanks for the review.

Do you think, it can be back-ported after to previous release branch? I think the first release with this change is v2.7.0

[CatherineF-dev]

QQ: want to know more about the context. What are the impacts of not backporting into v2.7.0?

Some new features can't be used or the binary will be broken.

[clamoriniere]

QQ: want to know more about the context. What are the impacts of not backporting into v2.7.0?

Some new features can't be used or the binary will be broken.

Sure,

For context, we are vendoring part of the "kube-state-metrics" code in the datadog-agent (more details about why we are doing it in this blog post).
Currently we are vendoring kube-state-metrics version v2.4.2, but we discover a bug with Ingresses resource that is fixed in v2.7.0. So our initial tentative was to update to the last release version v2.8.0, but this version now vendor k8s.io/api v0.26.1 but our datadog-agent project only vendor v0.25.5, and we know that if we upgrade to to v0.26.0 now, we will break some compatibility with some deprecated Kubernetes resources on old Kubernetes versions.

So we tried to vendor the last version that uses k8s.io/api v0.25.x and that also has the Ingresses bug fix which corresponds to the version v2.7.0. And it is how we also discover that the pkg/builder/Builder struct is not anymore compliant with the BuilderInterface since the same version.
Here is the full story 😄

[mrueg]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CatherineF-dev, clamoriniere, L3n41c, mrueg

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