otel: Value Updates and Logging

[schloerke]

Goal: Log reactive value updates as OTel log events.

Tasks

• Add logging helper to shiny/otel/_core.py:
  • emit_log(body, severity, attributes) - Emit OTel log record
• Modify Value._set() to log value updates
• Store value name at creation time for logging
• Include session ID in log attributes

Acceptance Criteria

• Value updates logged when SHINY_OTEL_COLLECT >= reactivity
• Logs include value name (e.g., "Set reactiveVal myValue")
• Logs include session.id attribute
• No logs when collection level insufficient
• No-op when OTel SDK not configured

Files to Modify

• shiny/otel/_core.py
• shiny/reactive/_reactives.py (line ~180)