Alerting: Introduces /api/v1/ngalert/alertmanagers to expose discovered and dropped Alertmanager(s) (#37632)

* Alerting: Expose discovered and dropped Alertmanagers

Exposes the API for discovered and dropped Alertmanagers.

* make admin config poll interval configurable

* update after rebase

* wordsmith

* More wordsmithing

* change name of the config

* settings package too

pkg/setting/setting.go

@@ -405,6 +405,9 @@ type Cfg struct {
 	// Geomap base layer config
 	GeomapDefaultBaseLayerConfig map[string]interface{}
 	GeomapEnableCustomBaseLayers bool
+
+	// Unified Alerting
+	AdminConfigPollInterval time.Duration
 }
 
 // IsLiveConfigEnabled returns true if live should be able to save configs to SQL tables
@@ -898,6 +901,10 @@ func (cfg *Cfg) Load(args *CommandLineArgs) error {
 		return err
 	}
 
+	if err := cfg.readUnifiedAlertingSettings(iniFile); err != nil {
+		return err
+	}
+
 	explore := iniFile.Section("explore")
 	ExploreEnabled = explore.Key("enabled").MustBool(true)
 
@@ -1349,6 +1356,13 @@ func readRenderingSettings(iniFile *ini.File, cfg *Cfg) error {
 	return nil
 }
 
+func (cfg *Cfg) readUnifiedAlertingSettings(iniFile *ini.File) error {
+	ua := iniFile.Section("unified_alerting")
+	s := ua.Key("admin_config_poll_interval_seconds").MustInt(60)
+	cfg.AdminConfigPollInterval = time.Second * time.Duration(s)
+	return nil
+}
+
 func readAlertingSettings(iniFile *ini.File) error {
 	alerting := iniFile.Section("alerting")
 	AlertingEnabled = alerting.Key("enabled").MustBool(true)