feat(telemetry): add user.id to gen_ai.user.message log records

[Achuth17]

Please note that this change adds user.id field to an existing opt-in: OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT. user.id is no longer set as a span attribute; it is emitted only on log records.

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: #issue_number
• Related: #issue_number

2. Or, if no issue exists, describe the change:

Problem:

• user.id was being attached as a span attribute on generate_content spans, which conflicted with security/privacy guidance that prompt/response-related PII should not be put in spans.
• user.id should only be emitted on event logs (gated by the existing OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT opt-in), not on spans.

Solution:
What: Stop setting user.id as a span attribute. Instead, emit it only on gen_ai.user.message log records (and on the experimental operation_details log), gated by the existing OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT opt-in modes that include event output (EVENT_ONLY, SPAN_AND_EVENT).

Why this solution:

• Reuses an existing, well-known opt-in (OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT) rather than introducing a new one, keeps the user-facing surface small.
• Keeps PII (user.id) out of spans entirely.
• Threads a separate log_only_* parameter through the existing helpers so the span vs. log split is explicit at every call site, avoiding accidental leakage back into span attributes.
• Gracefully degrades when opentelemetry-instrumentation-google-genai doesn't yet support the event-only context key (via try/except ImportError), so the change is non-blocking on the upstream contrib PR.

Testing Plan

Tests run:

• tests/unittests/telemetry/test_spans.py — 35/35 passed. Includes new ported tests covering: user.id absent from span attributes, present on gen_ai.user.message logs only when OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT includes event output and user_id is non-null, genai-instrumentor branch doesn't receive user.id, and graceful degradation when the upstream opentelemetry-instrumentation-google-genai doesn't expose the event-only context key.
• tests/unittests/telemetry/ (broader suite incl. e2e test_node_functional.py) — 103/103 passed.
• tests/unittests/flows/ (regression on tracing.py callers) — 379/379 passed.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Manual End-to-End (E2E) Tests:

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Add any other context or screenshots about the feature request here.

[xuanyang15]

Merged by eb379be