[v10.0.x] Alerting: Support concurrent queries for saving alert instances (#70869)

* Alerting: Support concurrent queries for saving alert instances (#70525)

This commit adds support for concurrent queries when saving alert
instances to the database. This is an experimental feature in
response to some customers experiencing delays between rule evaluation
and sending alerts to Alertmanager, resulting in flapping. It is
disabled by default.

(cherry picked from commit 7edbe72483)

* remove changes in api_testing.go

---------

Co-authored-by: George Robinson <george.robinson@grafana.com>

pkg/setting/setting_unified_alerting.go

@@ -92,6 +92,8 @@ type UnifiedAlertingSettings struct {
 	Screenshots                   UnifiedAlertingScreenshotSettings
 	ReservedLabels                UnifiedAlertingReservedLabelSettings
 	StateHistory                  UnifiedAlertingStateHistorySettings
+	// MaxStateSaveConcurrency controls the number of goroutines (per rule) that can save alert state in parallel.
+	MaxStateSaveConcurrency int
 }
 
 type UnifiedAlertingScreenshotSettings struct {
@@ -350,6 +352,8 @@ func (cfg *Cfg) ReadUnifiedAlertingSettings(iniFile *ini.File) error {
 	}
 	uaCfg.StateHistory = uaCfgStateHistory
 
+	uaCfg.MaxStateSaveConcurrency = ua.Key("max_state_save_concurrency").MustInt(1)
+
 	cfg.UnifiedAlerting = uaCfg
 	return nil
 }