[Discover] Fix filtering out custom meta fields of Elasticsearch plugins enhanced documents (#137147)

* Restoring the ability of showing custom meta fields like _size

src/plugins/data/common/search/tabify/tabify_docs.test.ts

@@ -187,5 +187,41 @@ describe('tabify_docs', () => {
       const table = tabifyDocs(response);
       expect(table).toMatchSnapshot();
     });
+
+    it('handles custom meta fields provided by ES plugins correctly', () => {
+      // @ts-expect-error not full inteface
+      const responsePlugin = {
+        hits: {
+          hits: [
+            {
+              _id: 'hit-id-value',
+              _index: 'hit-index-value',
+              _type: 'hit-type-value',
+              _size: 12,
+              _score: 77,
+              _source: {},
+            },
+          ],
+        },
+      } as estypes.SearchResponse<unknown>;
+      const dataView = new DataView({
+        spec: {
+          id: 'test-index',
+          fields: {
+            sourceTest: {
+              name: 'sourceTest',
+              type: 'number',
+              searchable: true,
+              aggregatable: true,
+            },
+          },
+        },
+        metaFields: ['_id', '_index', '_score', '_type', '_size'],
+        fieldFormats: fieldFormats as any,
+      });
+      const table = tabifyDocs(responsePlugin, dataView);
+
+      expect(table.columns.map((col) => col.id)).toEqual(['_id', '_index', '_score', '_size']);
+    });
   });
 });

src/plugins/data/common/search/tabify/tabify_docs.ts

@@ -11,40 +11,8 @@ import { isPlainObject } from 'lodash';
 import { Datatable, DatatableColumn, DatatableColumnType } from '@kbn/expressions-plugin/common';
 import type { DataView } from '@kbn/data-views-plugin/common';
 
-type ValidMetaFieldNames = keyof Pick<
-  estypes.SearchHit,
-  | '_id'
-  | '_ignored'
-  | '_index'
-  | '_node'
-  | '_primary_term'
-  | '_routing'
-  | '_score'
-  | '_seq_no'
-  | '_shard'
-  | '_source'
-  | '_version'
->;
-const VALID_META_FIELD_NAMES: ValidMetaFieldNames[] = [
-  '_id',
-  '_ignored',
-  '_index',
-  '_node',
-  '_primary_term',
-  '_routing',
-  '_score',
-  '_seq_no',
-  '_shard',
-  '_source',
-  '_version',
-];
-
-function isValidMetaFieldName(field: string): field is ValidMetaFieldNames {
-  // Since the array above is more narrowly typed than string[], we cannot use
-  // string to find a value in here. We manually cast it to wider string[] type
-  // so we're able to use `includes` on it.
-  return (VALID_META_FIELD_NAMES as string[]).includes(field);
-}
+// meta fields we won't merge with our result hit
+const EXCLUDED_META_FIELDS: string[] = ['_type', '_source'];
 
 interface TabifyDocsOptions {
   shallow?: boolean;
@@ -138,12 +106,12 @@ export function flattenHit(hit: Hit, indexPattern?: DataView, params?: TabifyDoc
   }
 
   // Merge all valid meta fields into the flattened object
-  // expect for _source (in case that was specified as a meta field)
-  indexPattern?.metaFields?.forEach((metaFieldName) => {
-    if (!isValidMetaFieldName(metaFieldName) || metaFieldName === '_source') {
+  indexPattern?.metaFields?.forEach((fieldName) => {
+    const isExcludedMetaField = EXCLUDED_META_FIELDS.includes(fieldName) || fieldName.at(0) !== '_';
+    if (isExcludedMetaField) {
       return;
     }
-    flat[metaFieldName] = hit[metaFieldName];
+    flat[fieldName] = hit[fieldName as keyof estypes.SearchHit];
   });
 
   // Use a proxy to make sure that keys are always returned in a specific order,