MM-8607 Add ability to turn off non-critical services when under load (#13212)

* MM-8607: add ability to turn off non-critical services under load

* server busy invalid param unit tests

* MM-8607: rename server busy endpoints

* MM-8607: handle case where App not initialized

* MM-8607: additional unit test cases per feedback.

* MM-8607: use decorator to check isbusy when adding endpoint route

* MM-8607: rename endpoints, use struct for json

* Update api4/system.go

Fix misspelled log output

Co-Authored-By: Saturnino Abril <saturnino.abril@gmail.com>

* MM-8607: fix i18n order; max seconds for server busy expiry

api4/handlers.go

@@ -107,3 +107,23 @@ func (api *API) ApiSessionRequiredTrustRequester(h func(*Context, http.ResponseW
 	return handler
 
 }
+
+// DisableWhenBusy provides a handler for API endpoints which should be disabled when the server is under load,
+// responding with HTTP 503 (Service Unavailable).
+func (api *API) ApiSessionRequiredDisableWhenBusy(h func(*Context, http.ResponseWriter, *http.Request)) http.Handler {
+	handler := &web.Handler{
+		GetGlobalAppOptions: api.GetGlobalAppOptions,
+		HandleFunc:          h,
+		HandlerName:         web.GetHandlerName(h),
+		RequireSession:      true,
+		TrustRequester:      false,
+		RequireMfa:          false,
+		IsStatic:            false,
+		DisableWhenBusy:     true,
+	}
+	if *api.ConfigService.Config().ServiceSettings.WebserverMode == "gzip" {
+		return gziphandler.GzipHandler(handler)
+	}
+	return handler
+
+}