[ML] API test for ml_node_count (#187484)

We can't be sure of the node count when running tests, so we just make
sure the counts are above expected values.

Also updates the route access tags to be `access:ml:canGetMlInfo` rather
than `access:ml:canGetJobs` and `access:ml:canGetDatafeeds`.
In serverless, AD can be disabled and these tags would be false.

x-pack/plugins/ml/server/routes/system.ts

@@ -147,7 +147,7 @@ export function systemRoutes(
       path: `${ML_INTERNAL_BASE_PATH}/ml_node_count`,
       access: 'internal',
       options: {
-        tags: ['access:ml:canGetJobs', 'access:ml:canGetDatafeeds'],
+        tags: ['access:ml:canGetMlInfo'],
       },
     })
     .addVersion(

x-pack/test/api_integration/apis/ml/system/index.ts

@@ -12,5 +12,6 @@ export default function ({ loadTestFile }: FtrProviderContext) {
     loadTestFile(require.resolve('./capabilities'));
     loadTestFile(require.resolve('./space_capabilities'));
     loadTestFile(require.resolve('./index_exists'));
+    loadTestFile(require.resolve('./node_count'));
   });
 }

x-pack/test/api_integration/apis/ml/system/node_count.ts

@@ -0,0 +1,44 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import expect from '@kbn/expect';
+
+import { FtrProviderContext } from '../../../ftr_provider_context';
+import { USER } from '../../../../functional/services/ml/security_common';
+import { getCommonRequestHeader } from '../../../../functional/services/ml/common_api';
+
+export default ({ getService }: FtrProviderContext) => {
+  const supertest = getService('supertestWithoutAuth');
+  const ml = getService('ml');
+
+  async function runRequest(user: USER, expectedStatusCode: number) {
+    const { body, status } = await supertest
+      .get(`/internal/ml/ml_node_count`)
+      .auth(user, ml.securityCommon.getPasswordForUser(user))
+      .set(getCommonRequestHeader('1'));
+    ml.api.assertResponseStatusCode(expectedStatusCode, status, body);
+
+    return body;
+  }
+
+  describe('GET ml/ml_node_count', function () {
+    describe('get ml node count', () => {
+      it('should match expected values', async () => {
+        const resp = await runRequest(USER.ML_POWERUSER, 200);
+        expect(resp.count).to.be.greaterThan(0, 'count should be greater than 0');
+        expect(resp.lazyNodeCount).to.be.greaterThan(
+          -1,
+          'lazyNodeCount should be greater or equal to 0'
+        );
+      });
+
+      it('should should fail for a unauthorized user', async () => {
+        await runRequest(USER.ML_UNAUTHORIZED, 403);
+      });
+    });
+  });
+};