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/api/frontendsettings.go

@@ -207,6 +207,7 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *models.ReqContext) (map[string]i
 		"alertingErrorOrTimeout":     setting.AlertingErrorOrTimeout,
 		"alertingNoDataOrNullValues": setting.AlertingNoDataOrNullValues,
 		"alertingMinInterval":        setting.AlertingMinInterval,
+		"liveEnabled":                hs.Cfg.LiveMaxConnections != 0,
 		"autoAssignOrg":              setting.AutoAssignOrg,
 		"verifyEmailEnabled":         setting.VerifyEmailEnabled,
 		"sigV4AuthEnabled":           setting.SigV4AuthEnabled,