[Logs onboarding] Getting elastic-agent version from fleet (elastic#1…

…66920)

Closes elastic#165657.

In elastic#166811 we exposed the value of
latest agent version available, with this PR we are aiming to use that
value as the `elastic-agent` version used in the installation script of
onboarding flow.

### How to test
1. Enter [System logs
onboarding](https://yngrdyn-deploy-kiban-pr166920.kb.us-west2.gcp.elastic-cloud.com/app/observabilityOnboarding/systemLogs)
2. Verify the elastic agent version in the installation script
<img width="870" alt="image"
src="https://github.com/elastic/kibana/assets/1313018/c2d99244-6e5a-4d2e-a3fd-eea6041636d2">

3. Go to the
[console](https://yngrdyn-deploy-kiban-pr166920.kb.us-west2.gcp.elastic-cloud.com/app/dev_tools#/console)
4. Execute `GET /`
5. Verify Kibana version

As you can see the kibana version is an snapshot but the elastic agent
version proposed is the latest one released `8.10.1` allowing us to
construct a valid download url like
https://artifacts.elastic.co/downloads/beats/elastic-agent/8.10.1-linux-x86_64.tar.gz

x-pack/plugins/observability_onboarding/server/plugin.ts

@@ -46,7 +46,10 @@ export class ObservabilityOnboardingPlugin
   }
 
   public setup(
-    core: CoreSetup<ObservabilityOnboardingPluginStartDependencies>,
+    core: CoreSetup<
+      ObservabilityOnboardingPluginStartDependencies,
+      ObservabilityOnboardingPluginStart
+    >,
     plugins: ObservabilityOnboardingPluginSetupDependencies
   ) {
     this.logger.debug('observability_onboarding: Setup');

x-pack/plugins/observability_onboarding/server/routes/logs/route.ts

@@ -40,9 +40,23 @@ const installShipperSetupRoute = createObservabilityOnboardingServerRoute({
     scriptDownloadUrl: string;
     elasticAgentVersion: string;
   }> {
-    const { core, plugins } = resources;
+    const { core, plugins, kibanaVersion } = resources;
     const coreStart = await core.start();
 
+    const fleetPluginStart = await plugins.fleet.start();
+    const agentClient = fleetPluginStart.agentService.asInternalUser;
+
+    // If undefined, we will follow fleet's strategy to select latest available version:
+    // for serverless we will use the latest published version, for statefull we will use
+    // current Kibana version. If false, irrespective of fleet flags and logic, we are
+    // explicitly deciding to not append the current version.
+    const includeCurrentVersion = kibanaVersion.endsWith('-SNAPSHOT')
+      ? false
+      : undefined;
+
+    const elasticAgentVersion =
+      await agentClient.getLatestAgentAvailableVersion(includeCurrentVersion);
+
     const kibanaUrl =
       core.setup.http.basePath.publicBaseUrl ?? // priority given to server.publicBaseUrl
       plugins.cloud?.setup?.kibanaUrl ?? // then cloud id
@@ -53,7 +67,7 @@ const installShipperSetupRoute = createObservabilityOnboardingServerRoute({
     return {
       apiEndpoint,
       scriptDownloadUrl,
-      elasticAgentVersion: '8.9.1',
+      elasticAgentVersion,
     };
   },
 });

x-pack/plugins/observability_onboarding/server/types.ts

@@ -11,6 +11,10 @@ import {
   PluginSetup as DataPluginSetup,
   PluginStart as DataPluginStart,
 } from '@kbn/data-plugin/server';
+import {
+  FleetSetupContract,
+  FleetStartContract,
+} from '@kbn/fleet-plugin/server';
 import { ObservabilityPluginSetup } from '@kbn/observability-plugin/server';
 import { UsageCollectionSetup } from '@kbn/usage-collection-plugin/server';
 
@@ -19,13 +23,15 @@ export interface ObservabilityOnboardingPluginSetupDependencies {
   observability: ObservabilityPluginSetup;
   cloud: CloudSetup;
   usageCollection: UsageCollectionSetup;
+  fleet: FleetSetupContract;
 }
 
 export interface ObservabilityOnboardingPluginStartDependencies {
   data: DataPluginStart;
   observability: undefined;
   cloud: CloudStart;
   usageCollection: undefined;
+  fleet: FleetStartContract;
 }
 
 // eslint-disable-next-line @typescript-eslint/no-empty-interface

x-pack/plugins/observability_onboarding/tsconfig.json

@@ -32,7 +32,8 @@
     "@kbn/use-tracked-promise",
     "@kbn/custom-integrations",
     "@kbn/share-plugin",
-    "@kbn/deeplinks-observability"
+    "@kbn/deeplinks-observability",
+    "@kbn/fleet-plugin",
   ],
   "exclude": [
     "target/**/*",