tiup: document Prometheus external labels in Chinese

[zanmato1984]

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• 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

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 自定义外部标签（external_labels）配置说明。
  • 补充 environment、region 等配置示例。
  • 说明标签命名限制及 cluster、monitor 保留标签约束。
  • 明确该配置需 TiUP v1.17.0 及以上版本支持，旧版本解析时可能失败。

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📥 Commits

Reviewing files that changed from the base of the PR and between a260f2e and 7ddea36.

📒 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 v1.17.0 起的 Prometheus external_labels 配置文档，覆盖自定义指南、集群拓扑参考和 DM 拓扑参考，并说明标签限制、示例及旧版 TiUP 的解析失败行为。

Changes

Prometheus external labels 文档

Layer / File(s)	Summary
自定义配置说明 tiup/customized-montior-in-tiup-environment.md	新增 monitoring_servers.external_labels 的配置入口、YAML 示例、标签命名限制及版本兼容性说明。
集群与 DM 拓扑参考 tiup/tiup-cluster-topology-reference.md, tiup/tiup-dm-topology-reference.md	在字段说明和配置示例中补充 external_labels、environment 与 region，并说明保留标签、命名规则及旧版解析失败行为。

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

Suggested labels: missing-translation-status

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了这次对 TiUP Prometheus external_labels 中文文档的新增。
Description check	✅ Passed	描述包含必填的变更说明、影响版本、相关链接和 AI 参与信息，且与模板基本一致。
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

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• Create PR with unit tests

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[LGTM Timeline notifier]

Timeline:

• 2026-07-27 03:55:47.604511323 +0000 UTC m=+1809133.640606369: ☑️ agreed by lilin90.