[ES|QL] Add a helper to retrieve the metadata columns (#191814)

## Summary

Create a helper to retrieve the metadata columns (will come in handy for
one discover project)

### Checklist

- [ ] [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

packages/kbn-esql-utils/README.md

@@ -8,4 +8,4 @@ This package contains utilities for ES|QL.
 - *removeDropCommandsFromESQLQuery*: Use this function to remove all the occurences of the `drop` command from the query.
 - *appendToESQLQuery*: Use this function to append more pipes in an existing ES|QL query. It adds the additional commands in a new line. 
 - *appendWhereClauseToESQLQuery*: Use this function to append where clause in an existing query. 
-
+- *retieveMetadataColumns*: Use this function to get if there is a metadata option in the from command, and retrieve the columns if so

packages/kbn-esql-utils/index.ts

@@ -24,6 +24,7 @@ export {
   getTimeFieldFromESQLQuery,
   getStartEndParams,
   hasStartEndParams,
+  retieveMetadataColumns,
   TextBasedLanguages,
 } from './src';
 

packages/kbn-esql-utils/src/index.ts

@@ -16,6 +16,7 @@ export {
   removeDropCommandsFromESQLQuery,
   hasTransformationalCommand,
   getTimeFieldFromESQLQuery,
+  retieveMetadataColumns,
 } from './utils/query_parsing_helpers';
 export { appendToESQLQuery, appendWhereClauseToESQLQuery } from './utils/append_to_query';
 export {

packages/kbn-esql-utils/src/utils/query_parsing_helpers.test.ts

@@ -12,6 +12,7 @@ import {
   removeDropCommandsFromESQLQuery,
   hasTransformationalCommand,
   getTimeFieldFromESQLQuery,
+  retieveMetadataColumns,
 } from './query_parsing_helpers';
 
 describe('esql query helpers', () => {
@@ -175,4 +176,17 @@ describe('esql query helpers', () => {
       ).toBe('event.timefield');
     });
   });
+
+  describe('retieveMetadataColumns', () => {
+    it('should return metadata columns if they exist', () => {
+      expect(retieveMetadataColumns('from a  metadata _id, _ignored | eval b = 1')).toStrictEqual([
+        '_id',
+        '_ignored',
+      ]);
+    });
+
+    it('should return empty columns if metadata doesnt exist', () => {
+      expect(retieveMetadataColumns('from a | eval b = 1')).toStrictEqual([]);
+    });
+  });
 });

packages/kbn-esql-utils/src/utils/query_parsing_helpers.ts

@@ -5,7 +5,13 @@
  * in compliance with, at your election, the Elastic License 2.0 or the Server
  * Side Public License, v 1.
  */
-import type { ESQLSource, ESQLFunction, ESQLColumn, ESQLSingleAstItem } from '@kbn/esql-ast';
+import type {
+  ESQLSource,
+  ESQLFunction,
+  ESQLColumn,
+  ESQLSingleAstItem,
+  ESQLCommandOption,
+} from '@kbn/esql-ast';
 import { getAstAndSyntaxErrors, Walker, walk } from '@kbn/esql-ast';
 
 const DEFAULT_ESQL_LIMIT = 1000;
@@ -105,3 +111,14 @@ export const getTimeFieldFromESQLQuery = (esql: string) => {
 
   return column?.name;
 };
+
+export const retieveMetadataColumns = (esql: string): string[] => {
+  const { ast } = getAstAndSyntaxErrors(esql);
+  const options: ESQLCommandOption[] = [];
+
+  walk(ast, {
+    visitCommandOption: (node) => options.push(node),
+  });
+  const metadataOptions = options.find(({ name }) => name === 'metadata');
+  return metadataOptions?.args.map((column) => (column as ESQLColumn).name) ?? [];
+};