update system metadata service to be observable-first

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

packages/backend-plugin-api/src/services/definitions/SystemMetadataService.ts

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+import { Observable } from '@backstage/types';
+
 /**
  * @public
  */
@@ -26,5 +28,5 @@ export interface BackstageInstance {
  * @public
  */
 export interface SystemMetadataService {
-  introspect(): Promise<{ instances: BackstageInstance[] }>;
+  instances(): Observable<BackstageInstance[]>;
 }