feat: Add selector and bootstrap observability metrics

[rawadhossain]

Description

This PR adds three new metrics. Two metrics are fully implemented, while the third is left with TODOs pending discussion.

What each metric does

node_readiness_selector_matched_nodes_total

Tracks how many nodes currently match a rule's spec. If a rule's NodeSelector matches no nodes, controller performs no work and produces no other signal. This metric makes those misconfigurations immediately visible.

node_readiness_bootstrap_completion_errors_total

Counts failures writing the bootstrap completion annotation. If this write fails, the node continues to be re-evaluated even though bootstrap completed. This metric makes those failures visible.

node_readiness_bootstrap_nrc_duration_seconds

Measures only the time NRC itself held a node, from the first taint until bootstrap completion. Excludes pre-NRC boot time.

It's registered, but recording logic deferred pending discussion on the timestamp anchor.
The two approaches I see are:

Option A:

• Write readiness.k8s.io/taint-applied-<rule> to node.ObjectMeta when the taint is first applied and record duration from that timestamp to bootstrap completion.

Option B:

• Write a dedicated node status condition and use its API-server-generated lastTransitionTime as the start timestamp.
• Requires a separate Status().Patch() call and nodes/status write permissions.

I left TODOs, so implementation can be completed once we agree on the approach.

Related to Issue #182

Type of Change

/kind feature

Testing

• Added tests coverages.

Checklist

• make test passes
• make lint passes

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[kubernetes-prow]

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This pull-request has been approved by: rawadhossain
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[kubernetes-prow]

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