[Misc] add iteration_tokens metric

[LucasWilkinson]

Add a metric to track the number of tokens per iteration in a histogram, useful for getting an idea of import GEMM shapes.

Metric is taken from: #4464 which has gone stale so credit to @HarryWu99 for parts of the code

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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🚀

[LucasWilkinson]

/ready

[github-actions]

This pull request 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 pull request should remain open. Thank you!

[github-actions]

This pull request has been automatically closed due to inactivity. Please feel free to reopen if you intend to continue working on it. Thank you!