server: refactor sleep handling, allow access /metrics during sleep (#27376)

* add cached responses

* refactor on_sleeping_state

* allow accessing metrics during sleep

* metrics task should not reset timer

* updated docs

* fix

* fix get_res_model_info

* add test

* fix a race condition

* split metrics and slots tasks / results

* should_reset_buckets

tools/server/README.md

@@ -2071,6 +2071,7 @@ Note that the following endpoints are exempt from being considered as incoming t
 - `GET /health`
 - `GET /props`
 - `GET /models`
+- `GET /metrics`
 
 ## More examples