experimental_telemetry metadata removed during LLM refactor

[Raviguntakala]

Description

The userId and sessionId metadata added in #5279 was removed during the LLM code rework.

What happened:

• PR #5279 added userId and sessionId to experimental_telemetry.metadata for session-level trace grouping
• During subsequent LLM refactoring, the llm.ts file was rewritten and lost the metadata fields
• Only isEnabled remains, breaking trace grouping in observability platforms

Expected behavior: experimental_telemetry should include metadata.userId and metadata.sessionId in all AI SDK calls.

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Terminal

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