Live: max_connections option with strict default (#34634)

this should help Live to be enabled by default but still
do not affect setups with lots of simultenious users. To
properly handle many WS connections Grafana administrators
should tune infrastructure a bit - for example increase a
number of open files for a process. Will be in more details
in documentation.

pkg/setting/setting.go

@@ -380,6 +380,11 @@ type Cfg struct {
 	ExpressionsEnabled bool
 
 	ImageUploadProvider string
+
+	// LiveMaxConnections is a maximum number of WebSocket connections to
+	// Grafana Live ws endpoint (per Grafana server instance). 0 disables
+	// Live, -1 means unlimited connections.
+	LiveMaxConnections int
 }
 
 // IsLiveConfigEnabled returns true if live should be able to save configs to SQL tables
@@ -950,6 +955,10 @@ func (cfg *Cfg) Load(args *CommandLineArgs) error {
 	cfg.readDateFormats()
 	cfg.readSentryConfig()
 
+	if err := cfg.readLiveSettings(iniFile); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1420,3 +1429,12 @@ func (cfg *Cfg) readDataSourcesSettings() {
 	datasources := cfg.Raw.Section("datasources")
 	cfg.DataSourceLimit = datasources.Key("datasource_limit").MustInt(5000)
 }
+
+func (cfg *Cfg) readLiveSettings(iniFile *ini.File) error {
+	section := iniFile.Section("live")
+	cfg.LiveMaxConnections = section.Key("max_connections").MustInt(100)
+	if cfg.LiveMaxConnections < -1 {
+		return fmt.Errorf("unexpected value %d for [live] max_connections", cfg.LiveMaxConnections)
+	}
+	return nil
+}