[ISSUE #8354] Make prometheus client optional in nacos-client SDK

[EvanYao826]

What does this PR do?

Fixes #8354: Make the nacos-client SDK lighter by making the prometheus dependency optional.

Problem

nacos-client has a hard dependency on io.prometheus:simpleclient. Users who don't need metrics monitoring still pull in this dependency, which adds to the SDK footprint and can cause version conflicts.

Solution

1. Mark io.prometheus:simpleclient as <optional>true</optional> in client/pom.xml
2. Refactor MetricsMonitor to detect prometheus availability at runtime via Class.forName(). When prometheus is not on the classpath, all monitoring operations become no-ops.
3. Extract prometheus API calls into PrometheusMetricsHelper (only loaded when prometheus is available)
4. Replace direct Histogram.Child usage in MetricsHttpAgent with a MetricsTimer interface
5. Update all callers (ClientWorker, ServiceInfoHolder, NamingGrpcClientProxy) to use the new simplified API

Changes

File	Change
client/pom.xml	Add <optional>true</optional> to prometheus dependency
MetricsMonitor.java	Rewrite with runtime prometheus detection and no-op fallback
PrometheusMetricsHelper.java	New file: isolates all prometheus direct API calls
MetricsHttpAgent.java	Use MetricsTimer instead of direct Histogram.Child
ClientWorker.java	Use recordListenConfigCount()
ServiceInfoHolder.java	Use recordServiceInfoMapSize()
NamingGrpcClientProxy.java	Use recordNamingRequestFailed()

Backward Compatibility

• Users who already include prometheus in their classpath see no behavior change
• Users without prometheus get no-op monitoring instead of ClassNotFoundException

Assisted-by: Hermes Agent

[github-actions]

Thanks for your this PR. 🙏
Please check again for your PR changes whether contains any usage/api/configuration change such as Add new API , Add new configuration, Change default value of configuration.
If so, please add or update documents(markdown type) in docs/next/ for repository nacos-group/nacos-group.github.io

---

感谢您提交的PR。 🙏
请再次查看您的PR内容，确认是否包含任何使用方式/API/配置参数的变更，如：新增API、新增配置参数、修改默认配置等操作。
如果是，请确保在提交之前，在仓库nacos-group/nacos-group.github.io中的docs/next/目录下添加或更新文档（markdown格式）。

[github-actions]

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[CLAassistant]

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[EvanYao826]

I have read the CLA Document and I hereby sign the CLA