chore: Change default version to Alertmanager APIv2 (#13263)

docs/sources/shared/configuration.md

@@ -4233,9 +4233,10 @@ storage:
 # CLI flag: -ruler.alertmanager-refresh-interval
 [alertmanager_refresh_interval: <duration> | default = 1m]
 
-# If enabled requests to Alertmanager will utilize the V2 API.
+# Use Alertmanager APIv2. APIv1 was deprecated in Alertmanager 0.16.0 and is
+# removed as of 0.27.0.
 # CLI flag: -ruler.alertmanager-use-v2
-[enable_alertmanager_v2: <boolean> | default = false]
+[enable_alertmanager_v2: <boolean> | default = true]
 
 # List of alert relabel configs.
 [alert_relabel_configs: <relabel_config...>]

pkg/ruler/base/ruler.go

@@ -168,7 +168,7 @@ func (cfg *Config) RegisterFlags(f *flag.FlagSet) {
 	f.StringVar(&cfg.AlertmanagerURL, "ruler.alertmanager-url", "", "Comma-separated list of Alertmanager URLs to send notifications to. Each Alertmanager URL is treated as a separate group in the configuration. Multiple Alertmanagers in HA per group can be supported by using DNS resolution via '-ruler.alertmanager-discovery'.")
 	f.BoolVar(&cfg.AlertmanagerDiscovery, "ruler.alertmanager-discovery", false, "Use DNS SRV records to discover Alertmanager hosts.")
 	f.DurationVar(&cfg.AlertmanagerRefreshInterval, "ruler.alertmanager-refresh-interval", alertmanagerRefreshIntervalDefault, "How long to wait between refreshing DNS resolutions of Alertmanager hosts.")
-	f.BoolVar(&cfg.AlertmanangerEnableV2API, "ruler.alertmanager-use-v2", false, "If enabled requests to Alertmanager will utilize the V2 API.")
+	f.BoolVar(&cfg.AlertmanangerEnableV2API, "ruler.alertmanager-use-v2", true, "Use Alertmanager APIv2. APIv1 was deprecated in Alertmanager 0.16.0 and is removed as of 0.27.0.")
 	f.IntVar(&cfg.NotificationQueueCapacity, "ruler.notification-queue-capacity", alertmanagerNotificationQueueCapacityDefault, "Capacity of the queue for notifications to be sent to the Alertmanager.")
 	f.DurationVar(&cfg.NotificationTimeout, "ruler.notification-timeout", alertmanagerNotificationTimeoutDefault, "HTTP timeout duration when sending notifications to the Alertmanager.")