[Easy] Add str repr for IterationStats

[22quinn]

Purpose

for debugging

Test Plan

>>> from vllm.v1.metrics.stats import IterationStats
INFO 10-04 15:05:08 [__init__.py:215] Automatically detected platform cuda.
>>> a = IterationStats()
>>> a
IterationStats(iteration_timestamp=1759615513.896626, num_generation_tokens=0, num_prompt_tokens=0, num_preempted_reqs=0, finished_requests=[], max_num_generation_tokens_iter=[], n_params_iter=[], time_to_first_tokens_iter=[], inter_token_latencies_iter=[], waiting_lora_adapters={}, running_lora_adapters={})
>>> 

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request introduces a __repr__ method to the IterationStats class, which is a helpful addition for debugging purposes. The implementation is straightforward and effectively serializes the object's attributes into a string representation. I find no issues with this change.

[houseroad]

better to add some unittest