[ICU-621] Add ExperimentalGroupUnreadChannels setting (#8127)

* Add ExperimentalGroupUnreadChannels setting

* track ExperimentalGroupUnreadChannels in diagnostics

utils/config.go

@@ -397,6 +397,7 @@ func GenerateClientConfig(c *model.Config, diagnosticId string) map[string]strin
 	props["EnablePreviewFeatures"] = strconv.FormatBool(*c.ServiceSettings.EnablePreviewFeatures)
 	props["EnableTutorial"] = strconv.FormatBool(*c.ServiceSettings.EnableTutorial)
 	props["ExperimentalEnableDefaultChannelLeaveJoinMessages"] = strconv.FormatBool(*c.ServiceSettings.ExperimentalEnableDefaultChannelLeaveJoinMessages)
+	props["ExperimentalGroupUnreadChannels"] = strconv.FormatBool(*c.ServiceSettings.ExperimentalGroupUnreadChannels)
 
 	props["SendEmailNotifications"] = strconv.FormatBool(c.EmailSettings.SendEmailNotifications)
 	props["SendPushNotifications"] = strconv.FormatBool(*c.EmailSettings.SendPushNotifications)