Add OpenTelemetry / session metrics export

[PureWeen]

Summary

The Copilot CLI supports OpenTelemetry for traces and metrics. PolyPilot could export session metrics for observability dashboards.

Current CLI Support

The CLI emits OTel traces and metrics for session activity (tool calls, token usage, duration). This is visible in the CLI's public help and configuration.

Proposed Implementation

1. Metrics export — expose session metrics (tokens, tool calls, duration) via OTel
2. Dashboard integration — optional export to Grafana, Datadog, etc.
3. In-app metrics — lightweight metrics view within PolyPilot

Priority: 🟢 LOW