server: refactor + correctness fixes for metrics (#26920)

server: refactor + correctness fixes for metrics (#26920)

* server: refactor metrics

* move most fields to server_slot_stats

* cont

* rm result_timings

* tie stats to batch

* cont

* nits: move place in code

* exclude first generated token

* more accurate batch metrics tracking

* n_predict --> n_gen

* metrics_on_prediction

* metrics_flush_idle

* metrics: seperate cache/processed prompt tokens

* refactor server_task_result_metrics

* add test

* nits

* fix flush before reset()

* cont

* rm dead code

* nits