Clean up JSON output

[sjmonson]

There is a lot of minor improvements that can be made to the JSON output of guidellm. For example:

1. Deduplicate the prompt field for each request
2. Flatten some field structures (e.g. "decode_times": { "data": [] } -> "decode_times": [] ).
3. Label percentiles (e.g. "request_latency_percentiles": [ 1, 2,... ] -> "request_latency_percentiles": { "p01": 1, "p05": 5,. ... })
4. Give max and min with all percentiles
5. Drop concurrences timestamps (possibly replace with percentiles/min/max/mean)

[markurtz]

Feel free to dive in, but wanted to callout that there will be some restructuring along these lines for the output standardization in case there is duplicate work. Will share something out a bit later

[rgreenberg1]

Let's also make sure that the output JSON is storing all of the metadata we may want to know:
Model_name
Quantized (None, INT4, INT8, FP8)
Hardware
Inference Scenario
vllm version
vllm-config file (need the file)

GuideLLM results:
Tokens per Second 
Time to First Token (TTFT) 
Inter-token Latency (ITL)
End-to-End Request Latency (e2e_latency)
Requests Per Second (RPS) Profiles/Sweeps
Cost to generate a million output tokens (Internal) - Future

[markurtz]

#91 fixes the issues @sjmonson brought up within this issue. @rgreenberg1 can we move the extra pieces you included here into it's own issue?