feat(telemetry): implement Prometheus metrics catalog and HTTP metrics server

[archittmittal]

Objective & Scope

Implement the Prometheus metrics registry and background HTTP server in src/telemetry/metrics.py.

Technical Details & Implementation Plan

• Counters: chunkguard_bytes_downloaded_total, chunkguard_chunk_retries_total
• Histograms: chunkguard_chunk_download_duration_seconds, chunkguard_chunk_throughput_bytes_per_second
• Gauges: chunkguard_active_workers, chunkguard_download_progress_ratio
• Background HTTP server exposing /metrics

Deliverable Files

• src/telemetry/metrics.py
• tests/unit/test_metrics.py

Acceptance Criteria & Checkpoints

• Metric generation and serialization verified in unit tests
• Minimal performance overhead (< 1% CPU)

Parent Epic

Part of Epic #36