[Feature Flags] ECS-compliant logger (#214231)

## Summary

The OpenFeature clients receive a logger, but it logs errors like
`log('something went wrong', error)`. Our core logger then removes the
`error.message` as it prefers the message provided as the first
argument.

This PR wraps the logger to make sure that we handle this type of usage.

cc @pmuellr as he found out about this bug


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

src/core/packages/feature-flags/server-internal/src/create_open_feature_logger.test.ts

@@ -0,0 +1,52 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import type { Logger as OpenFeatureLogger } from '@openfeature/server-sdk';
+import { type MockedLogger, loggerMock } from '@kbn/logging-mocks';
+import { createOpenFeatureLogger } from './create_open_feature_logger';
+
+const LOG_LEVELS = ['debug', 'info', 'warn', 'error'] as const;
+
+describe('createOpenFeatureLogger', () => {
+  let kbnLogger: MockedLogger;
+  let openFeatureLogger: OpenFeatureLogger;
+
+  beforeEach(() => {
+    kbnLogger = loggerMock.create();
+    openFeatureLogger = createOpenFeatureLogger(kbnLogger);
+  });
+
+  test.each(LOG_LEVELS)('should log.%s() a simple message', (logLevel) => {
+    openFeatureLogger[logLevel]('message');
+    expect(kbnLogger[logLevel]).toHaveBeenCalledWith('message', { extraArguments: [] });
+  });
+
+  test.each(LOG_LEVELS)('should log.%s() a message with 1 argument (non-error)', (logLevel) => {
+    openFeatureLogger[logLevel]('message', 'something else');
+    expect(kbnLogger[logLevel]).toHaveBeenCalledWith('message', {
+      extraArguments: ['something else'],
+    });
+  });
+
+  test.each(LOG_LEVELS)(
+    'should log.%s() a message with 1 argument (error) in their expected ECS field',
+    (logLevel) => {
+      const error = new Error('Something went wrong');
+      openFeatureLogger[logLevel]('An error occurred:', error);
+      expect(kbnLogger[logLevel]).toHaveBeenCalledWith('An error occurred:', { error });
+    }
+  );
+
+  test.each(LOG_LEVELS)('should log.%s() a message with additional arguments', (logLevel) => {
+    openFeatureLogger[logLevel]('message', 'something else', 'another thing', { foo: 'bar' });
+    expect(kbnLogger[logLevel]).toHaveBeenCalledWith('message', {
+      extraArguments: ['something else', 'another thing', { foo: 'bar' }],
+    });
+  });
+});

src/core/packages/feature-flags/server-internal/src/create_open_feature_logger.ts

@@ -0,0 +1,50 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import type { Logger as OpenFeatureLogger } from '@openfeature/server-sdk';
+import type { Logger, LogMeta } from '@kbn/logging';
+
+interface LogMetaWithExtraArguments extends LogMeta {
+  extraArguments: unknown[];
+}
+
+function normalizeLogArguments(
+  logger: Logger,
+  logLevel: 'debug' | 'info' | 'warn' | 'error',
+  message: string,
+  ...args: unknown[]
+) {
+  // Special case when calling log('something went wrong', error)
+  if (args.length === 1 && args[0] instanceof Error) {
+    logger[logLevel](message, { error: args[0] });
+  } else {
+    logger[logLevel]<LogMetaWithExtraArguments>(message, { extraArguments: args });
+  }
+}
+
+/**
+ * The way OpenFeature logs messages is very similar to the console.log approach,
+ * which is not compatible with our LogMeta approach. This can result in our log removing information like any 3rd+
+ * arguments passed or the error.message when using log('message', error).
+ *
+ * This wrapper addresses this by making it ECS-compliant.
+ * @param logger The Kibana logger
+ */
+export function createOpenFeatureLogger(logger: Logger): OpenFeatureLogger {
+  return {
+    debug: (message: string, ...args: unknown[]) =>
+      normalizeLogArguments(logger, 'debug', message, ...args),
+    info: (message: string, ...args: unknown[]) =>
+      normalizeLogArguments(logger, 'info', message, ...args),
+    warn: (message: string, ...args: unknown[]) =>
+      normalizeLogArguments(logger, 'warn', message, ...args),
+    error: (message: string, ...args: unknown[]) =>
+      normalizeLogArguments(logger, 'error', message, ...args),
+  };
+}

src/core/packages/feature-flags/server-internal/src/feature_flags_service.ts

@@ -25,6 +25,7 @@ import {
 import deepMerge from 'deepmerge';
 import { filter, switchMap, startWith, Subject } from 'rxjs';
 import { get } from 'lodash';
+import { createOpenFeatureLogger } from './create_open_feature_logger';
 import { setProviderWithRetries } from './set_provider_with_retries';
 import { type FeatureFlagsConfig, featureFlagsConfig } from './feature_flags_config';
 
@@ -56,7 +57,7 @@ export class FeatureFlagsService {
   constructor(private readonly core: CoreContext) {
     this.logger = core.logger.get('feature-flags-service');
     this.featureFlagsClient = OpenFeature.getClient();
-    OpenFeature.setLogger(this.logger.get('open-feature'));
+    OpenFeature.setLogger(createOpenFeatureLogger(this.logger.get('open-feature')));
   }
 
   /**