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Measures the startup cost of enabling runtime metrics: init() wires up the native-metrics collector, the GC PerformanceObserver, the event-loop monitor, the dogstatsd client and the flush interval. The control vs with-metrics delta is that wiring cost. (The old idle-window shape timed a 1s flush window rather than real work and read as ~16% jitter; the per-collection cost is sub-millisecond and would need the collector's private capture path exposed to measure directly.)