Update csv output format

[tgerdesnv]

Old format:

Statistic,avg,min,max,p99,p95,p90,p75,p50,p25
time_to_first_token(ns),4238735,3367978,6702240,6371118,5344958,5006259,4841394,4146648,3484484
inter_token_latency(ns),27202264,3849435,138324924,28283424,27737593,27469154,27067290,26979956,26926962
request_latency(ns),3363927003,3367978,14238834483,14091273510,13740917508,13692672723,3752510140,4846258,3612270
num_output_token,126,0,584,562,509,505,135,0,0
output_token_throughput(per sec),36.37
request_throughput(per sec),0.29

New format:

Metric,avg,min,max,p99,p95,p90,p75,p50,p25
Time to First Token (ns),4238735,3367978,6702240,6371118,5344958,5006259,4841394,4146648,3484484
Inter Token Latency (ns),27202264,3849435,138324924,28283424,27737593,27469154,27067290,26979956,26926962
Request Latency (ns),3363927003,3367978,14238834483,14091273510,13740917508,13692672723,3752510140,4846258,3612270
Num Output Token,126,0,584,562,509,505,135,0,0

Metric,Value
Output Token Throughput (per sec),36.37
Request Throughput (per sec),0.29

Also added unit testing to protect the output format

[dyastremsky]

Fantastic work, Tim! This looks great. There is an ongoing conversation, but once that is resolved, happy to approve from my end.

[nv-hwoo]

one small nit. Otherwise, looks good overall!