server: benchmark: chat/completions scenario and other llm servers comparison (#5941)

* server: bench: Init a bench scenario with K6
See #5827

* server: bench: EOL EOF

* server: bench: PR feedback and improved k6 script configuration

* server: bench: remove llamacpp_completions_tokens_seconds as it include prompt processing time and it's misleading

server: bench: add max_tokens from SERVER_BENCH_MAX_TOKENS

server: bench: increase truncated rate to 80% before failing

* server: bench: fix doc

* server: bench: change gauge custom metrics to trend

* server: bench: change gauge custom metrics to trend
server: bench: add trend custom metrics for total tokens per second average

* server: bench: doc add an option to debug http request

* server: bench: filter dataset too short and too long sequences

* server: bench: allow to filter out conversation in the dataset based on env variable

* server: bench: fix assistant message sent instead of user message

* server: bench: fix assistant message sent instead of user message

* server : add defrag thold parameter

* server: bench: select prompts based on the current iteration id not randomly to make the bench more reproducible

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>