Add formatting to json file with metrics (#372)

## Summary

It's inconvenient to look at metrics.

## Details

-


## Test Plan

- code launch

## Related Issues

- Resolves ##371

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: markurtz

src/guidellm/benchmark/output.py

@@ -144,10 +144,9 @@ def save_json(self, path: Union[str, Path]) -> Path:
             )
 
         model_dict = self.model_dump()
-        model_json = json.dumps(model_dict)
 
-        with path.open("w") as file:
-            file.write(model_json)
+        with path.open("w", encoding="utf-8") as file:
+            json.dump(model_dict, file, ensure_ascii=False, indent=4)
 
         return path