Fix AggConfig backslash escaping (#193932)

## Summary

Fixes `AggConfig` handling to properly escape backslash as well as the
single quote it already escapes.

(I believe this has been around ever since
#2486.)

src/plugins/data/common/search/aggs/metrics/lib/get_response_agg_config_class.test.ts

@@ -0,0 +1,34 @@
+/*
+ * 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 { getResponseAggId } from './get_response_agg_config_class';
+
+describe('getResponseAggConfigClass', () => {
+  describe('getResponseAggId', () => {
+    it('should generate a dot-separated ID from parent/key', () => {
+      const id = getResponseAggId('parent', 'child');
+      expect(id).toBe('parent.child');
+    });
+
+    it('should use brackets/quotes if the value includes a dot', () => {
+      const id = getResponseAggId('parent', 'foo.bar');
+      expect(id).toBe(`parent['foo.bar']`);
+    });
+
+    it('should escape quotes', () => {
+      const id = getResponseAggId('parent', `foo.b'ar`);
+      expect(id).toBe(`parent['foo.b\\'ar']`);
+    });
+
+    it('should escape backslashes', () => {
+      const id = getResponseAggId('parent', `f\\oo.b'ar`);
+      expect(id).toBe(`parent['f\\\\oo.b\\'ar']`);
+    });
+  });
+});

src/plugins/data/common/search/aggs/metrics/lib/get_response_agg_config_class.ts

@@ -31,6 +31,15 @@ export interface IResponseAggConfig extends IMetricAggConfig {
   parentId: IMetricAggConfig['id'];
 }
 
+export function getResponseAggId(parentId: string, key: string) {
+  const subId = String(key);
+  if (subId.indexOf('.') > -1) {
+    return parentId + "['" + subId.replace(/[\\']/g, '\\$&') + "']"; // $& means the whole matched string
+  } else {
+    return parentId + '.' + subId;
+  }
+}
+
 export const create = (parentAgg: IMetricAggConfig, props: Partial<IMetricAggConfig>) => {
   /**
    * AggConfig "wrapper" for multi-value metric aggs which
@@ -41,17 +50,7 @@ export const create = (parentAgg: IMetricAggConfig, props: Partial<IMetricAggCon
    */
   function ResponseAggConfig(this: IResponseAggConfig, key: string) {
     const parentId = parentAgg.id;
-    let id;
-
-    const subId = String(key);
-
-    if (subId.indexOf('.') > -1) {
-      id = parentId + "['" + subId.replace(/'/g, "\\'") + "']";
-    } else {
-      id = parentId + '.' + subId;
-    }
-
-    this.id = id;
+    this.id = getResponseAggId(parentId, key);
     this.key = key;
     this.parentId = parentId;
   }