feat: 支持模板强制关闭智能体指标 --story=1070093903136959970

[benero]

Background

Generated Agent projects should be able to disable metric export locally even when the downloaded Agent configuration enables it.

Changes

• treat an explicitly configured BKAI_AGENT_ENABLE_METRICS value as the highest-priority metric switch
• default the generated template environment to BKAI_AGENT_ENABLE_METRICS=false
• document how to opt in to local metric validation
• cover the case where the environment disables metrics while Agent configuration and exporter credentials enable them

Validation

• plugin exporter and local observability tests: 43 passed
• changed-file lint, formatting, merge-conflict, and diff checks passed
• full repository pre-commit was executed; it remains blocked by existing lint findings outside this change

[tencentblueking-adm]

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