feat(metrics): emit Rust metrics through metrics-rs

[lvthanh03]

Summary

• emit sentry_arroyo Rust counters, gauges, and timers through the metrics-rs facade
• use the single recorder/exporter installed by the consuming application instead of a separate Arroyo metrics backend
• preserve existing metric names, labels, and millisecond timer values
• adapt the legacy StatsdRecorder API to the same metrics-rs global for backward compatibility

Applications own metrics initialization and install exactly one metrics-rs recorder/exporter. Arroyo only emits through that recorder and does not initialize or operate a separate exporter.

Test plan

• cargo fmt --all --check
• cargo check
• cargo test --test metrics_facade --test legacy_metrics_recorder
• cargo test --doc
• cargo test --all-targets with the six local-Kafka-dependent tests skipped (54 passed; CI provisions Kafka for the full suite)