fix(otel): fall back user.id onto span extras when EVENT_ONLY key is missing

[DmitriiK]

Summary

• When GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY cannot be imported from opentelemetry-instrumentation-google-genai, merge log_only_extra_attributes (including user.id) into GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY instead of silently dropping them.
• Preserves the EVENT_ONLY path when the constant exists.
• Fixes user.id silently dropped when GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY is missing #6361

Context

opentelemetry-instrumentation-google-genai through 1.0b0 only exports the span extras key. ADK’s except (ImportError, AttributeError): pass meant session.user_id never reached Cloud Trace / GenAI spans on the instrumentor path (Vertex Agent Engine).

Trade-off: user.id lands on generate_content spans when the event-only channel is unavailable (slightly broader than “log-only”), which is preferable to dropping identity entirely until instrumentation ships the EVENT_ONLY key (opentelemetry-python-genai#24).

Test plan

• With instrumentor active and EVENT_ONLY constant absent, generate_content spans include user.id from session.user_id
• With a mock that provides EVENT_ONLY key, behavior unchanged (attrs go to event-only channel)
• Manual: Agent Engine / local run + Trace Explorer filter user.id=<value> on generate_content spans

Made with Cursor

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[rohityan]

Hi @DmitriiK , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!

[DmitriiK]

@rohityan
submitted.
thanks