[INLONG-1055][Doc] Add documentation for developing user-defined flink metrics

[PeterZh6]

Fixes #1055

Motivation

This PR addresses the lack of documentation for user-defined Flink metrics in the inlong-sort component. Recent updates introduced new metrics capabilities, but there is limited guidance on how to add and integrate custom metrics, especially when creating a new connector. This documentation aims to provide clear development instructions to help developers more easily customize and extend metrics within inlong-sort.

Modifications

• Added a detailed guide on defining, registering, and utilizing custom Flink metrics in the inlong-sort framework.
• Included examples and code snippets for creating and integrating metrics with connectors, focusing on deserialization error tracking.
• Provided instructions on testing and verifying new metrics using sort-end-to-end-tests.
• Outlined potential issues and best practices, such as handling null pointers and ensuring correct metric registration.

Verifying this change

• Ensure that the documentation additions are aligned with the latest metrics capabilities and that they pass CI checks.

(Please pick either of the following options)

• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:

  • Documentation-only change, with no direct impact on code functionality.

• This change added tests and can be verified as follows:

  • N/A (Documentation-only PR)

Demo Screenshots of this change:
Chinese Version:

English Version:

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? N/A
• If a feature is not applicable for documentation, explain why? Documentation is the primary focus of this PR.
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation: N/A

[aloyszhang]

Suggestion, the doc file name change from sort_self_defined_metrics to custom_flink_metrics