[Hugging Face TGI] Add dashboard (DataDog#21394)

* Add dashboard

* Dashboard v1

* Dashboard v2

* Update request timeline widget to display per request time instead of sum

* Add LLM Observability group to dashboard

* Remove pause-auto-refresh from dashboard

* Add dark mode logo

* Implement suggestions from code review

* Add prefill and decode batch graphs

* Add dashboard image

* Fix manifest

* Reduce image size

Author: dkirov-dd