feat(telemetry): implement Prometheus metrics exporter and OpenTelemetry distributed tracing

[archittmittal]

Objective & Overview

Implement the production observability subsystem in ChunkGuard / ReliaDL, featuring a Prometheus metrics server and OpenTelemetry distributed tracing.

Technical Specification & Design

• Prometheus Metrics: Export counters, histograms, and gauges over /metrics HTTP endpoint.
• OpenTelemetry Tracing: Distributed tracing span hierarchy (download.session -> download.worker -> download.chunk -> storage.write).

Key Deliverables & Module Breakdown

• src/telemetry/metrics.py: Prometheus metrics registry and server
• src/telemetry/tracing.py: OpenTelemetry tracer and exporter setup

Acceptance Criteria & Checkpoints

• Prometheus metrics formatting conforming to OpenMetrics standard
• OTel trace span context properly propagated across coroutines

Documentation References

• Observability & Monitoring