v2.1.0

• Add telemetry counters to Async::Limiter::Limited and Async::Limiter::Queued: acquired_count, available_count, waiting_count, and reacquire_waiting_count for observability into limiter state.
• Add as_json and to_json methods to Async::Limiter::Generic for JSON serialization of limiter statistics.
• Fix unused variable warning in Async::Limiter::Timing::FixedWindow.