Overview

Replace the hand-rolled CacheExclusive<AtomicWord<long long>> counters in OpCounters with OTel-backed Counter<int64_t> instruments managed by MetricsService.

Scope of Work

• src/mongo/db/stats/opcounters.h/.cpp — Rewrite OpCounters struct to hold Counter<int64_t>& reference fields backed by MetricsService
• Various call sites — Migrate from getUpdate()->load() style to direct .value() access

Acceptance Criteria

• serverStatus.opcounters produces identical BSON output
• All existing gotInsert(), gotCommand(), etc. call sites unchanged
• opcounters exported natively over OTel pipeline