[Phase 7.1] Add Prometheus metrics for monitoring

[zhexuany]

Summary

Implement Prometheus metrics for monitoring conversion throughput, error rates, and resource utilization.

Parent Epic

• [Epic] Distributed Roboflow with Alibaba Cloud (OSS + ACK) #9 Distributed Roboflow with Alibaba Cloud

Dependencies

• Can start after Phase 3 complete
• Full integration after [Phase 9.1] Implement long-running Worker Deployment [READY TO START] #18 (Controller)

Tasks

7.1.1 Add Metrics Infrastructure

1. Add prometheus crate to dependencies
2. Create src/metrics/mod.rs
3. Define static metric registry
4. Create metrics initialization function
5. Feature-gate under metrics flag (optional)

7.1.2 Define Conversion Metrics

1. roboflow_episodes_processed_total - Counter with labels: dataset_id, status
2. roboflow_episode_processing_duration_seconds - Histogram
3. roboflow_frames_processed_total - Counter
4. roboflow_bytes_read_total - Counter with source label
5. roboflow_bytes_written_total - Counter with destination, type labels

7.1.3 Define Upload Metrics

1. roboflow_upload_duration_seconds - Histogram with file_type label
2. roboflow_upload_retries_total - Counter with reason label
3. roboflow_upload_bytes_total - Counter
4. roboflow_pending_uploads - Gauge

7.1.4 Define Resource Metrics

1. roboflow_buffer_usage_bytes - Gauge
2. roboflow_cache_hit_total - Counter
3. roboflow_cache_miss_total - Counter
4. roboflow_active_conversions - Gauge

7.1.5 Define Video Encoding Metrics

1. roboflow_video_encoding_duration_seconds - Histogram
2. roboflow_video_frames_encoded_total - Counter with camera label

7.1.6 Instrument Code

1. Add metrics to conversion loop
2. Add metrics to storage operations
3. Add metrics to upload coordinator
4. Add metrics to video encoder

7.1.7 Create Metrics Endpoint

1. Add axum dependency for HTTP server
2. Create /metrics endpoint
3. Create /health endpoint
4. Start metrics server in background

Acceptance Criteria

• All metrics defined and registered
• Conversion code instrumented
• Storage code instrumented
• Metrics endpoint serves Prometheus format
• Health endpoint works
• All tests pass

Files to Create

• src/metrics/mod.rs
• src/metrics/conversion.rs
• src/metrics/storage.rs
• src/metrics/server.rs