feat: add trace content config

[feng-95]

Feature: Add Trace Content Collection Config

Overview

This PR adds a VeADK tracing content collection switch. It allows users to disable input/output content collection for Agent, LLM, and tool calls while still keeping non-content trace data, such as span structure, model parameters, token usage, tool name, and latency.

Config example:

observability:
  opentelemetry:
    trace_content: false

Environment variable override:

OBSERVABILITY_OPENTELEMETRY_TRACE_CONTENT=false

Changes

Configuration

• Added OpenTelemetryConfig.trace_content, defaulting to true
• Reused the existing VeADK configuration flow
• Supports the unified priority order across system env vars, .env, and config.yaml

Tracing

• Added should_trace_content() as the centralized content tracing switch
• Added content attribute filtering in the attributes layer for LLM and tool spans
• Agent root span input/output content now follows the same config
• When disabled, LLM prompts, completions, messages, choices, and tool input/output are no longer added to spans

Metrics / Exporter

• APMPlus tool metrics skip tool token usage when gen_ai.tool.input/output is not present
• Non-content metrics such as span latency, tool name, and tool type are still preserved
• Avoids reading tool arguments or responses from the exporter when content tracing is disabled

Documentation

• Updated config.yaml.full
• Updated Chinese and English configuration docs, tracing docs, observability overview, and span attributes docs

Test

• ./.venv/bin/python -m pytest -q -p no:rerunfailures tests/test_tracing_content.py tests/test_tracing.py
• git diff --check

Covered scenarios:

• Content tracing is enabled by default
• Content tracing can be disabled through config or environment variables
• LLM, tool, and Agent root spans all skip input/output content when disabled
• Context override can temporarily enable content tracing
• APMPlus skips tool token usage when tool input/output content is missing, while keeping non-content metrics