Add E2E test and zero-code example for Google ADK / Gemini agent

[YaMachinu]

Problem

Google ADK is listed as a first-class supported framework, but unlike LangChain and Strands it has:

1. No zero-code example — only examples/adk_example_sdk.py exists; no adk_example_zero_code.py
2. 2. No E2E test — tests/test_e2e_live_agents.py covers only LangChain and Strands
      ADK uses a completely different span attribute schema (gcp.vertex.agent.*) compared to the GenAI semconv path. Without an E2E test, regressions in the AdkExtractor will ship undetected.

What's Needed

1. Zero-code example (examples/adk_example_zero_code.py)

GOOGLE_API_KEY=... OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318 \
  opentelemetry-instrument python examples/adk_example_zero_code.py

Following the pattern of langchain_example_zero_code.py.

2. E2E test class in tests/test_e2e_live_agents.py

@pytest.mark.skipif(not os.getenv("GOOGLE_API_KEY"), reason="requires GOOGLE_API_KEY")
class TestADKGeminiAgent:
    async def test_adk_agent_evaluation(self):
        # Run the ADK example, verify gcp.vertex.agent.* spans are extracted,
        # confirm evaluation pipeline produces valid results
        ...

Why This Matters

• Validates the AdkExtractor code path end-to-end
• • Demonstrates parity with LangChain/Strands coverage
• • • Provides a concrete integration reference for Vertex AI / Gemini users

[krisztianfekete]

Fixed in #51