feat: Add OpenTelemetry instrumentation for Google ADK (rebased from PR #57)

[Copilot]

Description

Rebases PR #57 changes onto the latest main branch after the main branch was replaced. The original PR targeted legacy/archived_main_20251113 which is now outdated.

Adds opentelemetry-instrumentation-google-adk package providing OpenTelemetry instrumentation for Google Agent Development Kit (ADK) applications following GenAI semantic conventions.

Key components:

• GoogleAdkInstrumentor - Auto-injects observability plugin into ADK Runners
• Span creation for agent invocations, LLM calls, and tool executions
• Standard GenAI metrics: gen_ai.client.operation.duration and gen_ai.client.token.usage
• Attribute extraction following OTel GenAI conventions (gen_ai.operation.name, gen_ai.provider.name, etc.)

from opentelemetry.instrumentation.google_adk import GoogleAdkInstrumentor

GoogleAdkInstrumentor().instrument(
    tracer_provider=your_tracer_provider,
    meter_provider=your_meter_provider
)

Fixes #35

Type of change

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

How Has This Been Tested?

• Unit tests for utility functions (test_utils.py)
• Integration tests for metrics with InMemoryMetricReader (test_metrics.py)
• Integration tests for spans with InMemorySpanExporter (test_plugin_integration.py)
• CodeQL security scan - no alerts

Does This PR Require a Core Repo Change?

• No.

Checklist:

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

Original prompt

#57 我现在有一个 PR 要合并，但是 main 分支被替换换掉了，现在需要重新向 最新的 main 分支提交一个 PR，请你帮忙完成这个任务，可能会有冲突，如果有冲突的要解决下

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