[Infra] Limit the number of metrics accepted by Snapshot API (#188181)

part of [3628](https://github.com/elastic/observability-dev/issues/3628)
- private


## Summary

After adding 20 items, users can no longer add more metrics and will see
the "Add metric" button disabled with a tooltip

<img width="1713" alt="image"
src="https://github.com/user-attachments/assets/c784b08b-e118-4491-b53d-46bfde898216">


### How to test

- Start a local Kibana instance pointing to an oblt cluster
- Navigate to Infrastructure
- Try to add more than 20 metrics in the Metrics dropdown.

x-pack/test/functional/apps/infra/home_page.ts

@@ -555,6 +555,39 @@ export default ({ getPageObjects, getService }: FtrProviderContext) => {
           });
         });
       });
+
+      it('should not allow adding more than 20 custom metrics', async () => {
+        // open
+        await pageObjects.infraHome.clickCustomMetricDropdown();
+
+        const fields = [
+          'process.cpu.pct',
+          'process.memory.pct',
+          'system.core.total.pct',
+          'system.core.user.pct',
+          'system.core.nice.pct',
+          'system.core.idle.pct',
+          'system.core.iowait.pct',
+          'system.core.irq.pct',
+          'system.core.softirq.pct',
+          'system.core.steal.pct',
+          'system.cpu.nice.pct',
+          'system.cpu.idle.pct',
+          'system.cpu.iowait.pct',
+          'system.cpu.irq.pct',
+        ];
+
+        for (const field of fields) {
+          await pageObjects.infraHome.addCustomMetric(field);
+        }
+        const metricsCount = await pageObjects.infraHome.getMetricsContextMenuItemsCount();
+        // there are 6 default metrics in the context menu for hosts
+        expect(metricsCount).to.eql(20);
+
+        await pageObjects.infraHome.ensureCustomMetricAddButtonIsDisabled();
+        // close
+        await pageObjects.infraHome.clickCustomMetricDropdown();
+      });
     });
 
     describe('alerts flyouts', () => {

x-pack/test/functional/page_objects/infra_home_page.ts

@@ -482,5 +482,27 @@ export function InfraHomePageProvider({ getService, getPageObjects }: FtrProvide
     async clickCloseFlyoutButton() {
       return testSubjects.click('euiFlyoutCloseButton');
     },
+
+    async clickCustomMetricDropdown() {
+      await testSubjects.click('infraInventoryMetricDropdown');
+    },
+
+    async addCustomMetric(field: string) {
+      await testSubjects.click('infraModeSwitcherAddMetricButton');
+      const groupByCustomField = await testSubjects.find('infraCustomMetricFieldSelect');
+      await comboBox.setElement(groupByCustomField, field);
+      await testSubjects.click('infraCustomMetricFormSaveButton');
+    },
+
+    async getMetricsContextMenuItemsCount() {
+      const contextMenu = await testSubjects.find('infraInventoryMetricsContextMenu');
+      const menuItems = await contextMenu.findAllByCssSelector('button.euiContextMenuItem');
+      return menuItems.length;
+    },
+
+    async ensureCustomMetricAddButtonIsDisabled() {
+      const button = await testSubjects.find('infraModeSwitcherAddMetricButton');
+      expect(await button.getAttribute('disabled')).to.be('true');
+    },
   };
 }