Address PR review comments

- Make ExtensionPointFactoryMiddleware an opaque type via new
  @internal/backend package using OpaqueType from @internal/opaque
- Use options object for createExtensionPointFactoryMiddleware
- Make middleware function async (returns Promise<T>)
- Remove extensionPointFactoryMiddleware from createBackend, keep only
  on createSpecializedBackend
- Export defaultServiceFactories from @backstage/backend-defaults

Signed-off-by: Jack Palmer <jackpalmer@spotify.com>

packages/backend-defaults/report.api.md

@@ -5,18 +5,14 @@
 ```ts
 import { Backend } from '@backstage/backend-app-api';
 import { BackendFeature } from '@backstage/backend-plugin-api';
-import { createExtensionPointFactoryMiddleware } from '@backstage/backend-app-api';
-import { ExtensionPointFactoryMiddleware } from '@backstage/backend-app-api';
+import { ServiceFactory } from '@backstage/backend-plugin-api';
 
 // @public (undocumented)
-export function createBackend(options?: {
-  extensionPointFactoryMiddleware?: ExtensionPointFactoryMiddleware[];
-}): Backend;
+export function createBackend(): Backend;
 
-export { createExtensionPointFactoryMiddleware };
+// @public (undocumented)
+export const defaultServiceFactories: ServiceFactory[];
 
 // @public
 export const discoveryFeatureLoader: BackendFeature;
-
-export { ExtensionPointFactoryMiddleware };
 ```

packages/backend-defaults/src/CreateBackend.ts

@@ -14,11 +14,8 @@
  * limitations under the License.
  */
 
-import {
-  Backend,
-  createSpecializedBackend,
-  ExtensionPointFactoryMiddleware,
-} from '@backstage/backend-app-api';
+import { Backend, createSpecializedBackend } from '@backstage/backend-app-api';
+import { ServiceFactory } from '@backstage/backend-plugin-api';
 import { auditorServiceFactory } from '@backstage/backend-defaults/auditor';
 import { authServiceFactory } from '@backstage/backend-defaults/auth';
 import { cacheServiceFactory } from '@backstage/backend-defaults/cache';
@@ -46,7 +43,8 @@ import {
 } from '@backstage/backend-defaults/alpha';
 import { instanceMetadataServiceFactory } from './alpha/entrypoints/instanceMetadata/instanceMetadataServiceFactory';
 
-export const defaultServiceFactories = [
+/** @public */
+export const defaultServiceFactories: ServiceFactory[] = [
   auditorServiceFactory,
   authServiceFactory,
   cacheServiceFactory,
@@ -80,11 +78,6 @@ export const defaultServiceFactories = [
 /**
  * @public
  */
-export function createBackend(options?: {
-  extensionPointFactoryMiddleware?: ExtensionPointFactoryMiddleware[];
-}): Backend {
-  return createSpecializedBackend({
-    defaultServiceFactories,
-    extensionPointFactoryMiddleware: options?.extensionPointFactoryMiddleware,
-  });
+export function createBackend(): Backend {
+  return createSpecializedBackend({ defaultServiceFactories });
 }

packages/backend-defaults/src/index.ts

@@ -20,7 +20,5 @@
  * @packageDocumentation
  */
 
-export { createBackend } from './CreateBackend';
-export type { ExtensionPointFactoryMiddleware } from '@backstage/backend-app-api';
-export { createExtensionPointFactoryMiddleware } from '@backstage/backend-app-api';
+export { createBackend, defaultServiceFactories } from './CreateBackend';
 export { discoveryFeatureLoader } from './discoveryFeatureLoader';