Update ADK doc according to issue #1173 - 3 - Cloud Trace Data Capture

[adk-bot]

3. Summary of the change

Doc file: docs/observability/cloud-trace.md

Current state:

The documentation does not mention environment variables for controlling data capture in traces.

Proposed Change:

Add a section "Privacy and Data Capture" explaining how to control the logging of sensitive content in traces.

Explain two environment variables:

1. ADK_CAPTURE_MESSAGE_CONTENT_IN_SPANS: Set to 'false' or '0' to disable capturing potentially PII data (LLM requests/responses/tool args) in ADK spans. Defaults to 'true'.
2. OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT: Set to 'true' or '1' to enable logging of prompt/response content in GenAI instrumented spans.

Also mention that ADK tracing now aligns with OpenTelemetry Semantic Conventions for Generative AI.

Reasoning:
New tracing implementation introduces environment variables to control data capture, which is critical for privacy and compliance. Users need to know how to configure this.

Reference: src/google/adk/telemetry/tracing.py

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