Expose some counters to nbd and network pools

[djeebus]

Note

Adds OTEL metrics for nbd/network pools and simplifies APIs by removing MeterProvider parameters, updating call sites accordingly.

• Telemetry/metrics:
  • NBD pool (internal/sandbox/nbd/pool.go):
    • Add OTEL counters: slots_pool.ready (up/down), slots_pool.acquired, slots_pool.released.
    • Track acquisitions, releases, and ready count; change release to accept ctx.
  • Network pool (internal/sandbox/network/pool.go):
    • Add OTEL counters: slots_pool.new/reused (up/down), slots_pool.acquired (with pool attr), slots_pool.returned, slots_pool.released.
    • Use new counters in Populate, Get, Return, cleanup, Close; cleanup/Close now accept ctx.
  • Shared telemetry (shared/pkg/telemetry/meters.go):
    • Remove pool-specific UpDownCounter names now defined locally in pools.
• API changes:
  • nbd.NewDevicePool() no longer takes a MeterProvider.
  • network.NewPool(newSize, reusedSize, nodeID, config) no longer takes a MeterProvider.
  • Update all call sites (benchmarks, CLI, orchestrator main, mock-nbd) to new signatures.

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[linear]

ENG-3148 Expose metrics on nbd usage