[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

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[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!

[github-actions]

This issue has been automatically closed due to inactivity. Please feel free to reopen if you feel it is still relevant. Thank you!