feat(metrics): add NodesByState, ReconciliationLatency, and BootstrapDuration metrics

[Shreya2005-2005]

Description

Add three Prometheus metrics missing from the controller, required for the SLO dashboard proposed in #182:

• node_readiness_nodes_by_state{rule, state}: gauge tracking per-rule node counts by readiness state (ready/not_ready)
• node_readiness_reconciliation_latency_seconds{rule, operation}: histogram tracking end-to-end latency of taint add/remove operations per rule
• node_readiness_bootstrap_duration_seconds{rule}: histogram tracking time elapsed from bootstrap start to completion per rule

Related Issue

Relates to #182

Type of Change

/kind feature

Testing

• go build ./... passes
• go test ./internal/... passes (45 controller specs + webhook specs)
• Verified all three metrics registered and wired at correct call sites in node_controller.go and nodereadinessrule_controller.go

Does this PR introduce a user-facing change?

Add three new Prometheus metrics: node_readiness_nodes_by_state,
node_readiness_reconciliation_latency_seconds, and
node_readiness_bootstrap_duration_seconds to support SLO dashboards
for the node-readiness-controller.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Shreya2005-2005
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[k8s-ci-robot]

Hi @Shreya2005-2005. Thanks for your PR.

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[Shreya2005-2005]

@ajaysundark whenever you get a chance review this