[FEATURE] Scrape-Time Metrics Collector

[rawadhossain]

Introduce the new scrape-time collector for metrics computed directly from the controller-runtime cache.

The collector will initially be used by node_readiness_rule_nodes and later extended to support the other metrics defined for the scrape-time collector.

Ref. New Scrape-Time Collector Surface

Metrics

• node_readiness_rule_nodes — implemented in feat: add scrape-time metrics collector #398
• node_readiness_blocked_nodes — implemented in feat: add blocked nodes metric to scrape-time collector #431
• node_readiness_rule_matched_nodes — migrate in a follow-up from feat: collector-based rule_matched_nodes and typed failure reasons #286

The plan is to implement the new scrape-time metrics incrementally, with the remaining metrics sharing the same ReadinessCollector introduced in #398.

[rawadhossain]

/assign @rawadhossain