Observability Task: indicator buried point documentation and case improvement（可观测性任务指标埋点与案例完善）

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

## Describe the proposal

English：Maybe a lot of monitoring indicators have been developed and improved in the past, and a beautiful dashboard has also been made, but this may still be difficult for Dubbo users. The more indicators, the more difficult it is to understand, so certain documents and cases are needed to make Dubbo users are easier to understand.

中文：可能前面开发和完善了很多的监控指标，也制作了精美的dashboard，但是这个对于Dubbo用户来说可能仍旧无从下手，指标项越多就越难以理解，所以需要一定的文档和案例等方式来让Dubbo用户更易于理解。

The issue names no files, tests, or entry points. Start by surveying the existing monitoring-indicator and dashboard documentation, then identify concrete user scenarios that need examples. Done means the scope is defined and user-facing documentation and cases explain how to understand and use the indicators.

Written by the indexing model from the issue text.