Allow benchmark to write json output

[richardsliu]

Modify the serving benchmark to write json output similar to Jetstream benchmark: https://github.com/richardsliu/JetStream/blob/vllm-benchmark/benchmarks/benchmark_serving.py#L648

This will allow the benchmark to integrate with XLML dashboard.

[achandrasekar]

Looks good. Thanks for adding this! Makes it easier to parse the results.