[Doc] Documentation Needed for eventmesh-connector-prometheus

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/eventmesh/issues?q=is%3Aissue) and found no similar issues.

### Documentation Related

Currently, most connectors lack documentation. The following details must be added to provide better clarity to the developers:- 1. functions of the current connector
2. descriptions of configuration items
3. precautions (if any),
4. relevant official reference links (if any),
5. Other specific content that needs to be explained in the documentation.
Having these details can increase the clarity for the users who are using the product and can also help in onboarding the new developers easily

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct) *

Start by locating the eventmesh-connector-prometheus implementation and its configuration entry points. Review the connector's functions and configuration, then document their usage, any precautions, and relevant official Prometheus references; done means developers can understand and use the connector from the documentation.

Written by the indexing model from the issue text.