[APM] Add feature flag for not available apm schema (elastic#158911)

Part of elastic#153776

How to test:

- change default value in x-pack/plugins/apm/common/apm_feature_flags.ts

x-pack/plugins/apm/public/components/routing/templates/settings_template.tsx

@@ -46,7 +46,9 @@ export function SettingsTemplate({ children, selectedTab }: Props) {
   const agentConfigurationAvailable = useApmFeatureFlag(
     ApmFeatureFlagName.AgentConfigurationAvailable
   );
-  const schemaAvailable = useApmFeatureFlag(ApmFeatureFlagName.SchemaAvailable);
+  const schemaTabAvailable = useApmFeatureFlag(
+    ApmFeatureFlagName.MigrationToFleetAvailable
+  );
   const indicesAvailable = useApmFeatureFlag(
     ApmFeatureFlagName.ConfigurableIndicesAvailable
   );
@@ -57,7 +59,7 @@ export function SettingsTemplate({ children, selectedTab }: Props) {
     router,
     defaultEnvironment,
     agentConfigurationAvailable,
-    schemaAvailable,
+    schemaTabAvailable,
     indicesAvailable,
   });
 
@@ -82,15 +84,15 @@ function getTabs({
   router,
   defaultEnvironment,
   agentConfigurationAvailable,
-  schemaAvailable,
+  schemaTabAvailable,
   indicesAvailable,
 }: {
   core: CoreStart;
   selectedTab: Tab['key'];
   router: ApmRouter;
   defaultEnvironment: Environment;
   agentConfigurationAvailable: boolean;
-  schemaAvailable: boolean;
+  schemaTabAvailable: boolean;
   indicesAvailable: boolean;
 }) {
   const canReadMlJobs = !!core.application.capabilities.ml?.canGetJobs;
@@ -169,7 +171,7 @@ function getTabs({
         ]
       : []),
 
-    ...(schemaAvailable
+    ...(schemaTabAvailable
       ? [
           {
             key: 'schema' as const,

x-pack/plugins/apm/server/routes/fleet/route.ts

@@ -13,6 +13,7 @@ import {
   APM_SERVER_SCHEMA_SAVED_OBJECT_ID,
   APM_SERVER_SCHEMA_SAVED_OBJECT_TYPE,
 } from '../../../common/apm_saved_object_constants';
+import { ApmFeatureFlags } from '../../../common/apm_feature_flags';
 import { createInternalESClientWithContext } from '../../lib/helpers/create_es_client/create_internal_es_client';
 import { getInternalSavedObjectsClient } from '../../lib/helpers/get_internal_saved_objects_client';
 import { createApmServerRoute } from '../apm_routes/create_apm_server_route';
@@ -30,6 +31,14 @@ import {
   RunMigrationCheckResponse,
 } from './run_migration_check';
 
+function throwNotFoundIfFleetMigrationNotAvailable(
+  featureFlags: ApmFeatureFlags
+): void {
+  if (!featureFlags.migrationToFleetAvailable) {
+    throw Boom.notFound();
+  }
+}
+
 const hasFleetDataRoute = createApmServerRoute({
   endpoint: 'GET /internal/apm/fleet/has_apm_policies',
   options: { tags: [] },
@@ -68,6 +77,7 @@ const saveApmServerSchemaRoute = createApmServerRoute({
     }),
   }),
   handler: async (resources): Promise<void> => {
+    throwNotFoundIfFleetMigrationNotAvailable(resources.featureFlags);
     const { params, logger, core } = resources;
     const coreStart = await core.start();
     const savedObjectsClient = await getInternalSavedObjectsClient(coreStart);
@@ -87,6 +97,7 @@ const getUnsupportedApmServerSchemaRoute = createApmServerRoute({
   handler: async (
     resources
   ): Promise<{ unsupported: UnsupportedApmServerSchema }> => {
+    throwNotFoundIfFleetMigrationNotAvailable(resources.featureFlags);
     const { context } = resources;
     const savedObjectsClient = (await context.core).savedObjects.client;
     return {
@@ -101,6 +112,8 @@ const getMigrationCheckRoute = createApmServerRoute({
   handler: async (resources): Promise<RunMigrationCheckResponse> => {
     const { core, plugins, context, config, request } = resources;
 
+    throwNotFoundIfFleetMigrationNotAvailable(resources.featureFlags);
+
     const { fleet, security } = plugins;
 
     if (!fleet || !security) {