add iLert plugin

Signed-off-by: yacut <roman.rogozhnikov@gmail.com>

plugins/ilert/README.md

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+# @backstage/plugin-ilert
+
+## Introduction
+
+[iLert](https://www.ilert.com) is a platform for alerting, on-call management and uptime monitoring. It helps teams to reduce response times to critical incidents by extending monitoring tools with reliable alerting, automatic escalations, on-call schedules and other features to support the incident response process, such as [informing stakeholders](https://docs.ilert.com/getting-started/stakeholder-engagement) or creating tickets in external incident management tools.
+
+## Overview
+
+This plugin displays iLert information about an entity such as if there are any active incidents, wo is on-call now and uptime monitor status.
+
+There is also an easy way to trigger an incident directly to the person who is currently on-call.
+
+This plugin provides:
+
+- Information details about the persons on-call
+- A way to override current person on-call
+- A list of incidents
+- A way to trigger a new incident
+- A way to reassign/acknowledge/resolve an incident
+- A list of uptime monitors
+
+## Setup instructions
+
+Install the plugin:
+
+```bash
+yarn add @backstage/plugin-ilert
+```
+
+Then make sure to export the plugin in your app's [`plugins.ts`](https://github.com/backstage/backstage/blob/master/packages/app/src/plugins.ts)
+to enable the plugin:
+
+```js
+export { plugin as ILert } from '@backstage/plugin-ilert';
+```
+
+Add it to the `EntityPage.tsx`:
+
+```ts
+import {
+  isPluginApplicableToEntity as isILertAvailable,
+  EntityILertCard,
+} from '@backstage/plugin-ilert';
+// add to code
+{
+  isILertAvailable(entity) && (
+    <Grid item md={6}>
+      <EntityILertCard />
+    </Grid>
+  );
+}
+```
+
+> To force iLert card for each entity just add the `<EntityILertCard />` component, so an instruction card will appears if no integration key is set.
+
+## Add iLert explorer to the App sidebar
+
+Modify your app routes in [`App.tsx`](https://github.com/backstage/backstage/blob/master/packages/app/src/App.tsx) to include the Router component exported from the plugin, for example:
+
+```tsx
+// At the top imports
+import { ILertPage } from '@backstage/plugin-ilert';
+
+// Inside App component
+<Routes>
+  // ...
+  <Route path="/ilert" element={<ILertPage />} />
+  // ...
+</Routes>;
+```
+
+Modify your sidebar in [`Root.tsx`](https://github.com/backstage/backstage/blob/master/packages/app/src/components/Root/Root.tsx) to include the icon component exported from the plugin, for example:
+
+```tsx
+// At the top imports
+import { ILertIcon } from '@backstage/plugin-ilert';
+
+// Inside Sidebar component
+<Sidebar>
+  // ...
+  <SidebarItem icon={ILertIcon} to="ilert" text="iLert" />
+  // ...
+</Sidebar>;
+```
+
+## Client configuration
+
+If you want to override the default URL for api calls and detail pages, you can add it to `app-config.yaml`.
+
+In `app-config.yaml`:
+
+```yaml
+ilert:
+  domain: https://my-org.ilert.com/
+```
+
+## Providing the Authorization Header
+
+In order to make the API calls, you need to provide a new proxy config which will redirect to the [iLert API](https://api.ilert.com/api-docs/) endpoint it needs an [Authorization Header](https://api.ilert.com/api-docs/#section/Authentication).
+
+Add the proxy configuration in `app-config.yaml`
+
+```yaml
+proxy:
+  ...
+  '/ilert':
+    target: https://api.ilert.com
+    allowedMethods: ['GET', 'POST', 'PUT']
+    allowedHeaders: ['Authorization']
+    headers:
+      Authorization:
+        $env: ILERT_AUTH_HEADER
+```
+
+Then start the backend passing the token as an environment variable:
+
+```bash
+$ ILERT_AUTH_HEADER='Basic <TOKEN>' yarn start
+```
+
+## Integration Key
+
+The information displayed for each entity is based on the [alert source integration key](https://docs.ilert.com/integrations/backstage).
+
+### Adding the integration key to the entity annotation
+
+If you want to use this plugin for an entity, you need to label it with the below annotation:
+
+```yml
+annotations:
+  ilert.com/integration-key: [INTEGRATION_KEY]
+```