Empty message for APM service map

When only one node is displayed, show an empty message.

Also:

* Start adding a basic Jest test for the ServiceMap component
* Fix bug where EuiDocsLink was rendering "children" instead of the actual children

Closes #59326.
Closes #59128.

x-pack/legacy/plugins/apm/public/components/app/ServiceMap/EmptyBanner.tsx

@@ -0,0 +1,43 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { EuiCallOut } from '@elastic/eui';
+import lightTheme from '@elastic/eui/dist/eui_theme_light.json';
+import { i18n } from '@kbn/i18n';
+import React from 'react';
+import styled from 'styled-components';
+import { ElasticDocsLink } from '../../shared/Links/ElasticDocsLink';
+
+const EmptyBannerCallOut = styled(EuiCallOut)`
+  margin: ${lightTheme.gutterTypes.gutterSmall};
+  /* Add some extra margin so it displays to the right of the controls. */
+  margin-left: calc(
+    ${lightTheme.gutterTypes.gutterLarge} +
+      ${lightTheme.gutterTypes.gutterExtraLarge}
+  );
+  position: absolute;
+  z-index: 1;
+`;
+
+export function EmptyBanner() {
+  return (
+    <EmptyBannerCallOut
+      title={i18n.translate('xpack.apm.serviceMap.emptyBanner.title', {
+        defaultMessage: "Looks like there's only a single service."
+      })}
+    >
+      {i18n.translate('xpack.apm.serviceMap.emptyBanner.message', {
+        defaultMessage:
+          "We will map out connected services and external requests if we can detect them. Please make sure you're running the latest version of the APM agent."
+      })}{' '}
+      <ElasticDocsLink section="/apm/get-started" path="/agents.html">
+        {i18n.translate('xpack.apm.serviceMap.emptyBanner.docsLink', {
+          defaultMessage: 'Learn more in the docs'
+        })}
+      </ElasticDocsLink>
+    </EmptyBannerCallOut>
+  );
+}

x-pack/legacy/plugins/apm/public/components/app/ServiceMap/index.test.tsx

@@ -0,0 +1,45 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { render } from '@testing-library/react';
+import React, { FunctionComponent } from 'react';
+import { License } from '../../../../../../../plugins/licensing/common/license';
+import { LicenseContext } from '../../../context/LicenseContext';
+import { MockApmPluginContextWrapper } from '../../../utils/testHelpers';
+import { ServiceMap } from './';
+
+const expiredLicense = new License({
+  signature: 'test signature',
+  license: {
+    expiryDateInMillis: 0,
+    mode: 'platinum',
+    status: 'expired',
+    type: 'platinum',
+    uid: '1'
+  }
+});
+
+const Wrapper: FunctionComponent = ({ children }) => {
+  return (
+    <LicenseContext.Provider value={expiredLicense}>
+      <MockApmPluginContextWrapper>{children}</MockApmPluginContextWrapper>
+    </LicenseContext.Provider>
+  );
+};
+
+describe('ServiceMap', () => {
+  describe('with an inactive license', () => {
+    it('renders the license banner', async () => {
+      expect(
+        (
+          await render(<ServiceMap />, {
+            wrapper: Wrapper
+          }).findAllByText(/Platinum/)
+        ).length
+      ).toBeGreaterThan(0);
+    });
+  });
+});

x-pack/legacy/plugins/apm/public/components/app/ServiceMap/index.tsx

@@ -26,13 +26,14 @@ import { useLicense } from '../../../hooks/useLicense';
 import { useLoadingIndicator } from '../../../hooks/useLoadingIndicator';
 import { useLocation } from '../../../hooks/useLocation';
 import { useUrlParams } from '../../../hooks/useUrlParams';
+import { callApmApi } from '../../../services/rest/createCallApmApi';
 import { Controls } from './Controls';
 import { Cytoscape } from './Cytoscape';
+import { EmptyBanner } from './EmptyBanner';
 import { getCytoscapeElements } from './get_cytoscape_elements';
 import { PlatinumLicensePrompt } from './PlatinumLicensePrompt';
 import { Popover } from './Popover';
 import { useRefDimensions } from './useRefDimensions';
-import { callApmApi } from '../../../services/rest/createCallApmApi';
 
 interface ServiceMapProps {
   serviceName?: string;
@@ -214,6 +215,9 @@ export function ServiceMap({ serviceName }: ServiceMapProps) {
         style={cytoscapeDivStyle}
       >
         <Controls />
+        {serviceName && renderedElements.current.length === 1 && (
+          <EmptyBanner />
+        )}
         <Popover focusedServiceName={serviceName} />
       </Cytoscape>
     </div>

x-pack/legacy/plugins/apm/public/components/shared/Links/ElasticDocsLink.tsx

@@ -23,7 +23,7 @@ export function ElasticDocsLink({ section, path, children, ...rest }: Props) {
     children(href)
   ) : (
     <EuiLink href={href} {...rest}>
-      children
+      {children}
     </EuiLink>
   );
 }