[google-genai] Provide a way to attach extra attributes to the operation-details event but not to the span

[Achuth17]

Fixes #4580

Description

Adds a sibling context key,
GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY, to
opentelemetry-instrumentation-google-genai for attaching caller-supplied
attributes that should be emitted only on the
gen_ai.client.inference.operation.details log event and never on the
generate_content {model} span.

The existing GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY (added in #3961
for spans, extended in #4103 for events) writes attributes to both the
span and the event with no opt-out. That makes it unsafe for caller-supplied
attributes that are acceptable on event log records but must not land on
broadly-sampled spans, for example, an end-user identifier (user.id).

Concrete motivating case: the [Google ADK][adk] passes a
common_attributes dict containing the USER_ID semconv constant
(string value "user.id", from
opentelemetry.semconv._incubating.attributes.user_attributes) through this
context key, which causes user.id to land on every generate_content span
as soon as opentelemetry-instrumentation-google-genai is installed. ADK
has no way today to keep that value off the span without also losing it from
the operation-details event.

The new key follows the exact same naming and access pattern as the
existing GENERATE_CONTENT_EXTRA_ATTRIBUTES_CONTEXT_KEY, keeping the
public API surface uniform. It is read in all four generate_content
wrappers (sync/async × streaming/non-streaming) and merged into the
LogRecord only — never into span.set_attributes(...). On key collisions
with the existing extras channel, the event-only value wins on the event;
the span carries the existing extra-attributes value.

No new semconv keys are introduced, keys remain caller-defined, like the
existing extras channel. No external dependencies added or changed.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Targeted unit tests (8 new test cases — 2 per wrapper, run for sync +
  async × streaming + non-streaming via the existing shared base classes
  in nonstreaming_base.py and streaming_base.py):

  • test_event_only_extra_attributes_not_set_on_span — verifies an
    attribute attached only via GENERATE_CONTENT_EVENT_ONLY_EXTRA_ATTRIBUTES_CONTEXT_KEY
    is absent from generate_content {model} span attributes.
  • test_event_only_extra_attributes_set_on_event_only — verifies the
    same attribute appears on the
    gen_ai.client.inference.operation.details event LogRecord, plus the
    collision-precedence rule (event-only value wins on the event; the
    existing-extras value is what reaches the span).

• Test commands run:

  # Targeted (event-only and existing extra-attributes tests, all 4 wrappers):
  uv run tox -e py312-test-instrumentation-google-genai-latest -- -k "event_only or extra_genai_attributes"
  # -> 24 passed, 24 skipped (intentional base-class skips), 0 failed
  
  # Full package suite (no regressions in pre-existing tests):
  uv run tox -e py312-test-instrumentation-google-genai-latest
  # -> 204 passed, 114 skipped, 0 failed
  
  # Static analysis:
  uv run tox -e typecheck
  # -> 0 errors, 0 warnings (pyright)
  
  # Lint + format:
  uvx ruff check  <touched files>
  uvx ruff format --check  <touched files>
  # -> All checks passed; 4 files already formatted

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

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[DylanRussell]

This looks good but should be opened in https://github.com/open-telemetry/opentelemetry-python-genai where we recently moved the instrumentations..

Also open-telemetry/opentelemetry-python-genai#10 -- you might wait until this is merged but I can also merge my PR on top of yours so that doesn't matter too much

[Achuth17]

Moved the PR to open-telemetry/opentelemetry-python-genai#24