[Metric threshold] Fix the condition not showing up in the metric thr…

…eshold flyout (elastic#192736)

Closes elastic#192439

## Summary

This PR reverts this
[line](https://github.com/elastic/kibana/pull/191948/files#diff-2dd82a791bba3d995e9e6b35d4a973053f166351cc6025a5cd1d24dc789766aeR48)
in a previous [PR](elastic#191948) that
caused an issue in loading the metric threshold flyout on the
observability alerts page.

| Before ❌ | After ✅ |
|---|---|

|![Image](https://github.com/user-attachments/assets/3c0b8812-8cd9-4769-bd20-ab10f559009b)|![Image](https://github.com/user-attachments/assets/9823e691-ce18-4c00-8748-ce5797a19943)|

I also added a small test that fails before this fix.

x-pack/plugins/observability_solution/infra/public/alerting/metric_threshold/components/expression_row.tsx

@@ -200,7 +200,7 @@ export const ExpressionRow = ({
 
   return (
     <>
-      <EuiFlexGroup gutterSize="xs">
+      <EuiFlexGroup gutterSize="xs" data-test-subj="metricThresholdExpressionRow">
         <EuiFlexItem grow={false}>
           <EuiButtonIcon
             iconType={isExpanded ? 'arrowDown' : 'arrowRight'}

x-pack/plugins/observability_solution/infra/public/containers/metrics_source/source.tsx

@@ -45,7 +45,7 @@ export const useSourceFetcher = ({ sourceId }: { sourceId: string }) => {
             method: 'GET',
           }
         );
-        telemetry.reportPerformanceMetricEvent(
+        telemetry?.reportPerformanceMetricEvent(
           'infra_source_load',
           performance.now() - start,
           {},

x-pack/test/functional/services/observability/alerts/rules_page.ts

@@ -6,6 +6,8 @@
  */
 import { FtrProviderContext } from '../../../ftr_provider_context';
 
+const METRIC_THRESHOLD_RULE_TYPE_SELECTOR = 'metrics.alert.threshold-SelectOption';
+
 export function ObservabilityAlertsRulesProvider({ getService }: FtrProviderContext) {
   const testSubjects = getService('testSubjects');
   const find = getService('find');
@@ -30,12 +32,25 @@ export function ObservabilityAlertsRulesProvider({ getService }: FtrProviderCont
     await find.clickByButtonText('metric-threshold');
   };
 
+  const clickOnInfrastructureCategory = async () => {
+    const categories = await testSubjects.find('ruleTypeModal');
+    const category = await categories.findByCssSelector(`.euiFacetButton[title="Infrastructure"]`);
+    await category.click();
+  };
+
+  const clickOnMetricThresholdRule = async () => {
+    await testSubjects.existOrFail(METRIC_THRESHOLD_RULE_TYPE_SELECTOR);
+    await testSubjects.click(METRIC_THRESHOLD_RULE_TYPE_SELECTOR);
+  };
+
   return {
     getManageRulesPageHref,
     clickCreateRuleButton,
     clickRuleStatusDropDownMenu,
     clickDisableFromDropDownMenu,
     clickLogsTab,
     clickOnRuleInEventLogs,
+    clickOnInfrastructureCategory,
+    clickOnMetricThresholdRule,
   };
 }

x-pack/test/observability_functional/apps/observability/index.ts

@@ -24,5 +24,6 @@ export default function ({ loadTestFile }: FtrProviderContext) {
     loadTestFile(require.resolve('./pages/rules_page'));
     loadTestFile(require.resolve('./pages/rule_details_page'));
     loadTestFile(require.resolve('./pages/alert_details_page'));
+    loadTestFile(require.resolve('./pages/alerts/metric_threshold'));
   });
 }

x-pack/test/observability_functional/apps/observability/pages/alerts/metric_threshold.ts

@@ -0,0 +1,38 @@
+/*
+ * 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 { FtrProviderContext } from '../../../../ftr_provider_context';
+
+export default ({ getService }: FtrProviderContext) => {
+  const esArchiver = getService('esArchiver');
+  const testSubjects = getService('testSubjects');
+
+  describe('Metric threshold rule', function () {
+    this.tags('includeFirefox');
+
+    const observability = getService('observability');
+
+    before(async () => {
+      await esArchiver.load('x-pack/test/functional/es_archives/infra/metrics_and_logs');
+      await observability.alerts.common.navigateToRulesPage();
+    });
+
+    after(async () => {
+      await esArchiver.unload('x-pack/test/functional/es_archives/infra/metrics_and_logs');
+    });
+
+    it('shows the metric threshold rule in the observability section', async () => {
+      await observability.alerts.rulesPage.clickCreateRuleButton();
+      await observability.alerts.rulesPage.clickOnInfrastructureCategory();
+      await observability.alerts.rulesPage.clickOnMetricThresholdRule();
+    });
+
+    it('shows an expression row in the condition section', async () => {
+      await testSubjects.existOrFail('metricThresholdExpressionRow');
+    });
+  });
+};