adding rollbar plugin & backend

This is the first commit of the rollbar plugin. It is fully functional
but will require some additional features to make it more usable and
feature complete. It currently includes a backend wraps the rollbar REST
API and provides data for the frontend plugin.

plugins/rollbar/README.md

@@ -0,0 +1,56 @@
+# Rollbar Plugin
+
+Website: [https://rollbar.com/](https://rollbar.com/)
+
+## Setup
+
+1. Configure the [rollbar backend plugin](https://github.com/spotify/backstage/tree/master/plugins/rollbar-backend/README.md)
+
+2. If you have standalone app (you didn't clone this repo), then do
+
+```bash
+yarn add @backstage/plugin-rollbar
+```
+
+3. Add plugin to the list of plugins:
+
+```ts
+// packages/app/src/plugins.ts
+export { plugin as Rollbar } from '@backstage/plugin-rollbar';
+```
+
+4. Add plugin API to your Backstage instance:
+
+```ts
+// packages/app/src/api.ts
+import { RollbarClient, rollbarApiRef } from '@backstage/plugin-rollbar';
+
+// ...
+
+builder.add(
+  rollbarApiRef,
+  new RollbarClient({
+    apiOrigin: backendUrl,
+    basePath: '/rollbar',
+  }),
+);
+
+// Alternatively you can use the mock client
+// builder.add(rollbarApiRef, new RollbarMockClient());
+```
+
+5. Run app with `yarn start` and navigate to `/rollbar`
+
+## Features
+
+- List rollbar projects
+- View top active items for each project
+
+## Limitations
+
+- Rollbar has rate limits per token
+
+## Links
+
+- (Backend part of the plugin)[https://github.com/spotify/backstage/tree/master/plugins/rollbar-backend]
+- (The Backstage homepage)[https://backstage.io]