fix: explicitly stringify extra fields passed to the logger service

Signed-off-by: Paul Schultz <pschultz@pobox.com>

.changeset/itchy-schools-camp.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-defaults': patch
+---
+
+Explicitly stringify extra logger fields with `JSON.stringify` to prevent `[object Object]` errors.

packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.test.ts

@@ -93,28 +93,4 @@ describe('WinstonLogger', () => {
       expect.any(Function),
     );
   });
-
-  it('gracefully handles fields that are not castable to a string', () => {
-    const mockTransport = new Transport({
-      log: jest.fn(),
-      logv: jest.fn(),
-    });
-
-    const logger = WinstonLogger.create({
-      transports: [mockTransport],
-    });
-
-    logger.error('something went wrong', {
-      field: Object.create(null),
-    });
-
-    expect(mockTransport.log).toHaveBeenCalledWith(
-      expect.objectContaining({
-        [MESSAGE]: expect.stringContaining(
-          '[field value not castable to string]',
-        ),
-      }),
-      expect.any(Function),
-    );
-  });
 });

packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.ts

@@ -151,7 +151,7 @@ export class WinstonLogger implements RootLoggerService {
             let stringValue = '';
 
             try {
-              stringValue = `${value}`;
+              stringValue = JSON.stringify(value);
             } catch (e) {
               stringValue = '[field value not castable to string]';
             }