feat(sdk): Add Prometheus exposition format export

[lowhung]

Summary

Add support for exporting metrics in Prometheus text-based exposition format, enabling scraping by Prometheus or compatible monitoring systems.

Proposed Solution

• Add new prometheus feature flag to buswatch-sdk
• Create prometheus.rs module with:
  • PrometheusConfig builder for configuration
  • PrometheusExporter for storing and rendering metrics
  • format_prometheus() function to convert snapshots to Prometheus format
• Add Output::prometheus() variant

Metrics to Export

• buswatch_read_count (counter) - Total messages read
• buswatch_write_count (counter) - Total messages written
• buswatch_read_backlog (gauge) - Unread message count
• buswatch_read_pending_seconds (gauge) - Read wait time
• buswatch_write_pending_seconds (gauge) - Write wait time
• buswatch_read_rate_per_second (gauge) - Read throughput
• buswatch_write_rate_per_second (gauge) - Write throughput

Labels

All metrics should include module and topic labels.

[lowhung]

This feature has been implemented in PR #48.

The implementation includes:

• prometheus feature flag in buswatch-sdk
• PrometheusConfig builder and PrometheusExporter
• Output::prometheus() variant for easy integration
• HTTP server (using hyper) serving metrics at configurable endpoint
• All proposed metrics with module and topic labels
• Bonus: /health and /healthz endpoints for Kubernetes probes

[lowhung]

Closing - implemented in PR #48.