[APM] Revert deletion of service agent test (elastic#200196)

Closes elastic#198988
Part of elastic#193245

## Summary

Follow up from elastic#199812. The
`agent.spec.ts` test was deleted by mistake.

### How to test

- Serverless

```
node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts --grep="APM"
```

It's recommended to be run against
[MKI](https://github.com/crespocarlos/kibana/blob/main/x-pack/test_serverless/README.md#run-tests-on-mki)

- Stateful
```
node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts --grep="APM"
```

- [ ] ~(OPTIONAL, only if a test has been unskipped) Run flaky test
suite~
- [x] local run for serverless
- [x] local run for stateful
- [x] MKI run for serverless

---------

Co-authored-by: kibanamachine <[email protected]>

x-pack/test/api_integration/deployment_agnostic/apis/observability/apm/services/agent.spec.ts

@@ -0,0 +1,65 @@
+/*
+ * 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 archives_metadata from '../../../../../../apm_api_integration/common/fixtures/es_archiver/archives_metadata';
+import type { DeploymentAgnosticFtrProviderContext } from '../../../../ftr_provider_context';
+import { ARCHIVER_ROUTES } from '../constants/archiver';
+
+export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderContext) {
+  const apmApiClient = getService('apmApi');
+  const esArchiver = getService('esArchiver');
+
+  const archiveName = '8.0.0';
+  const { start, end } = archives_metadata[archiveName];
+
+  describe('Agent name', () => {
+    describe('when data is not loaded', () => {
+      it('handles the empty state', async () => {
+        const response = await apmApiClient.readUser({
+          endpoint: 'GET /internal/apm/services/{serviceName}/agent',
+          params: {
+            path: { serviceName: 'opbeans-node' },
+            query: {
+              start,
+              end,
+            },
+          },
+        });
+
+        expect(response.status).to.be(200);
+        expect(response.body).to.eql({});
+      });
+    });
+
+    describe('when data is loaded', () => {
+      before(async () => {
+        await esArchiver.load(ARCHIVER_ROUTES[archiveName]);
+      });
+      after(async () => {
+        await esArchiver.unload(ARCHIVER_ROUTES[archiveName]);
+      });
+
+      it('returns the agent name', async () => {
+        const response = await apmApiClient.readUser({
+          endpoint: 'GET /internal/apm/services/{serviceName}/agent',
+          params: {
+            path: { serviceName: 'opbeans-node' },
+            query: {
+              start,
+              end,
+            },
+          },
+        });
+
+        expect(response.status).to.be(200);
+
+        expect(response.body).to.eql({ agentName: 'nodejs', runtimeName: 'node' });
+      });
+    });
+  });
+}

x-pack/test/api_integration/deployment_agnostic/apis/observability/apm/services/index.ts

@@ -13,6 +13,7 @@ export default function ({ loadTestFile }: DeploymentAgnosticFtrProviderContext)
     loadTestFile(require.resolve('./error_groups/error_groups_main_statistics.spec.ts'));
     loadTestFile(require.resolve('./service_details/service_details.spec.ts'));
     loadTestFile(require.resolve('./service_icons/service_icons.spec.ts'));
+    loadTestFile(require.resolve('./agent.spec.ts'));
     loadTestFile(require.resolve('./archive_services_detailed_statistics.spec.ts'));
     loadTestFile(require.resolve('./derived_annotations.spec.ts'));
     loadTestFile(require.resolve('./get_service_node_metadata.spec.ts'));

x-pack/test/apm_api_integration/common/fixtures/es_archiver/archives_metadata.ts

@@ -6,6 +6,10 @@
  */
 
 export default {
+  '8.0.0': {
+    start: '2020-08-26T11:00:43.849Z',
+    end: '2020-08-26T12:00:43.849Z',
+  },
   'apm_8.0.0': {
     start: '2021-08-03T06:50:15.910Z',
     end: '2021-08-03T07:20:15.910Z',