feat: add node_readiness_build_info metric

[rawadhossain]

Description

Adds node_readiness_build_info metric from observability design doc

Changes

• Add node_readiness_build_info gauge with a version label.
• Set the metric at controller startup using the build version.
• Wire build version and git commit metadata through -ldflags in the Makefile and Dockerfile.

Implementation choices

• Use the build version as the version label, while keeping commit information in the existing startup version log.
• Added -ldflags wiring because the existing internal/info version variables were not being populated during builds, which otherwise left the version as unknown.

Related to Issue #182

Type of Change

/kind feature

Verification

• Verified the metric reports the actual git-derived version from a locally built binary.
• Verified the same version is propagated through the Docker build path.

Checklist

• make test passes
• make lint passes

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[kubernetes-prow]

Hi @rawadhossain. Thanks for your PR.

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[Ayush4958]

Pls add a git_commit label here []string{"version", "git_commit"}

Prometheus standards require the commit hash on build metrics

[rawadhossain]

I kept the metric scoped to the binary version since the commit is already exposed in the startup log.

[ajaysundark]

Looks like there's an idiomatic way to report this: prometheus/client_golang#693

[rawadhossain]

Thanks for pointing this out, the example helped. Updated it.

[ajaysundark]

The other identifying labels from prometheus example (eg: revision / goversion) are useful and could be extended as labels here, but I'm not blocking this CL on this, you could followup on this after -
https://github.com/prometheus/common/blob/2fb5a7cecbffd4bf7dc88ff22712112e8cef8432/version/info.go#L42

[rawadhossain]

Thanks. I’ll do follow-up on this if needed to extend going ahead.

[ajaysundark]

Looks like there's an idiomatic way to report this: prometheus/client_golang#693

[ajaysundark]

/lgtm

[rawadhossain]

@ajaysundark added the tests. Should be good to merge.

[ajaysundark]

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ajaysundark, rawadhossain

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[ajaysundark]

/ok-to-test