feat(telemetry): Add userId and sessionId metadata to experimental_telemetry

[Raviguntakala]

Summary

Adds userId and sessionId to experimental_telemetry.metadata across all AI SDK calls, enabling session-level trace grouping in observability platforms.

Motivation

Currently, OpenCode emits OTEL traces for cost and token usage tracking, but lacks user/session context. This makes it difficult to:

• Group traces by user sessions
• Track conversation flows end-to-end
• Correlate multiple LLM calls within a single session

Testing

• Tested with Langfuse ✅ - traces correctly group by session and display user attribution
• No impact on existing functionality when OTEL is disabled