[Feature]: Record EPLB metric, can in /metric api get info

[lengrongfu]

🚀 The feature, motivation and pitch

In #18343 (comment) this comment, i think we can export some metrics, such as the weight of each expert, to understand the hot and cold distribution of experts, etc.

Alternatives

No response

Additional context

No response

Before submitting a new issue...

• Make sure you already searched for relevant issues, and asked the chatbot living at the bottom right corner of the documentation page, which can answer lots of frequently asked questions.

[lengrongfu]

/assign

[lengrongfu]

#19915

[hmellor]

@lengrongfu would you like me to assign this to you?

[lengrongfu]

Not needed for now. A contributor has submitted a related PR. If there are any subsequent changes, I will submit a PR, thanks.

[github-actions]

This issue has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this issue should remain open. Thank you!