tiup: document Prometheus external labels in Chinese (#21837)

[ti-chi-bot]

This is an automated cherry-pick of #21837

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

This PR adds the Chinese documentation for TiUP support for monitoring_servers[].external_labels.

It mirrors the English docs update by documenting the new field in the TiUP cluster and DM topology references, and by adding a dedicated section to the monitoring customization guide. It also keeps the compatibility note aligned with the implementation: older TiUP versions fail to parse the topology file instead of ignoring the field.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

• This PR is translated from: tiup: document Prometheus external labels docs#23358
• Other reference link(s):
  • Support modify external_labels in prometheus.yml for Prometheus tiup#1795
  • cluster: support Prometheus external labels in topology tiup#2719
  • https://github.com/pingcap/tiup/releases#release-v1.17.0

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• 文档
  • 新增 Prometheus 全局 external_labels 配置说明，支持在监控服务拓扑配置中设置环境、区域等标签。
  • 补充配置示例、标签命名限制及保留标签规则。
  • 明确该功能自 TiUP v1.17.0 起支持，旧版本解析包含该字段的拓扑配置时会失败。

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No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 598728f and 23ef650.

📒 Files selected for processing (3)

• tiup/customized-montior-in-tiup-environment.md
• tiup/tiup-cluster-topology-reference.md
• tiup/tiup-dm-topology-reference.md

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📝 Walkthrough

Walkthrough

新增 TiUP monitoring_servers.external_labels 文档，涵盖配置方式、标签命名与保留规则、版本兼容行为，以及 TiUP Cluster 和 TiUP DM 的拓扑示例。

Changes

Prometheus 外部标签文档

Layer / File(s)	Summary
配置行为与约束 tiup/customized-montior-in-tiup-environment.md, tiup/*topology-reference.md	说明 external_labels 的引入版本、全局配置渲染行为、标签命名规则、保留标签及旧版 TiUP 的解析失败行为。
拓扑配置示例 tiup/tiup-cluster-topology-reference.md, tiup/tiup-dm-topology-reference.md	在 TiUP Cluster 和 TiUP DM 的 monitoring_servers 示例中加入外部标签配置。

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• pingcap/docs-cn#21837：同样补充 TiUP monitoring_servers[].external_labels 的中文文档、示例及旧版解析行为。

Suggested labels: translation/from-docs

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了文档新增 Prometheus external_labels 说明这一主要改动。
Description check	✅ Passed	描述基本覆盖模板要求，包含变更说明、适用版本、相关链接和 AI 参与信息。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[lilin90]

/approve

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