fix(metrics): restore IndexMap for unbounded MetricSet to fix CPU regression (#25493)

* perf(metrics): restore IndexMap for unbounded MetricSet to fix CPU regression

* refactor(metrics): remove pop_lru from MetricSetInner, check Bounded in enforce_capacity_policy

* chore(changelog): add context on when the perf regression is noticeable

* refactor(metrics): add debug_assert for impossible Bounded state in enforce_capacity_policy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>