Disable the experimental metrics_entities plugin by default. (#115460)

This was default disabled in 7.15, but we needed a code change to maintain that (consistent) behavior.
# Conflicts:
#	x-pack/plugins/metrics_entities/server/index.ts

x-pack/plugins/metrics_entities/server/index.ts

@@ -4,9 +4,9 @@
  * 2.0; you may not use this file except in compliance with the Elastic License
  * 2.0.
  */
+import { schema } from '@kbn/config-schema';
 
 import { PluginConfigDescriptor, PluginInitializerContext } from '../../../../src/core/server';
-
 import { ConfigSchema } from './config';
 import { MetricsEntitiesPlugin } from './plugin';
 
@@ -22,3 +22,10 @@ export const plugin = (initializerContext: PluginInitializerContext): MetricsEnt
 };
 
 export { MetricsEntitiesPluginSetup, MetricsEntitiesPluginStart } from './types';
+
+export const config = {
+  schema: schema.object({
+    // This plugin is experimental and should be disabled by default.
+    enabled: schema.boolean({ defaultValue: false }),
+  }),
+};