feat: add extension point to add custom attributes to generate_content spans in google genai instrumentor.

[RKest]

Description

Introduces ability to add additional attributes to generate_content {model.name} spans to Google GenAI instrumentation.

Motivation behind this change is to add (non-semconv) gcp.vertex.agent.event_id attribute, to generate_content {model.name} spans, in order to correlate events with spans in google/adk-python.

Broader motivation is:

• Currently in ADK we trace inference in both generate_content and legacy call_llm spans.
• We want to migrate fully to generate_content spans and deprecate call_llm spans.
• However call_llm spans contain gcp.vertex.agent.event_id attribute, and we still want to correlate events with spans.

Note: Event in ADK is not synonymous with event in OTEL. In ADK, Event == LLM Request/Response or Tool Call/Result, in OTEL IIUC Event == Log.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Tested using:

• Automated unit tests.
• Using opentelemetry-instrumentor-google-genai off my fork in ADK, and successfully adding custom attributes to target span

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• 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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: RKest / name: Max Ind (d8cc094)