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/server-task.cpp

@@ -1512,10 +1512,15 @@ json server_task_result_error::to_json() {
 //
 // server_task_result_metrics
 //
-json server_task_result_metrics::to_json() {
+json server_task_result_slots::to_json() {
     return slots_data;
 }
 
+json server_task_result_metrics::to_json() {
+    // not used, /metrics renders prometheus text via to_metrics()
+    return json{};
+}
+
 // metrics definition: https://prometheus.io/docs/practices/naming/#metric-names
 std::string server_task_result_metrics::to_metrics() {
     const std::vector<metric_item> counters = {