Fix issue with single value for fields params (#157930)

## Summary

The interface for fields allow array of string see below but the
validation with `@kbn/config-schema` only allow string. This will allow
to pass fields as array.

<img width="355" alt="image"
src="04c099c6-6a84-49ef-af45-587efa8e508b">


### 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

src/plugins/data_views/server/routes/fields_for.test.ts

@@ -6,20 +6,20 @@
  * Side Public License, v 1.
  */
 
-import { parseMetaFields } from './fields_for';
+import { parseFields } from './fields_for';
 
 describe('_fields_for_wildcard', () => {
   describe('parseMetaFields', () => {
     it('should throw if receiving a string of comma-separated values', () => {
       const value = '_source,_id';
-      expect(() => parseMetaFields(value)).toThrowErrorMatchingInlineSnapshot(
+      expect(() => parseFields(value)).toThrowErrorMatchingInlineSnapshot(
         `"metaFields should be an array of field names, a JSON-stringified array of field names, or a single field name"`
       );
     });
 
     it('should parse a stringified list of values', () => {
       const value = JSON.stringify(['_source', '_id']);
-      const fields = parseMetaFields(value);
+      const fields = parseFields(value);
       expect(fields).toMatchInlineSnapshot(`
       Array [
         "_source",
@@ -30,7 +30,7 @@ describe('_fields_for_wildcard', () => {
 
     it('should wrap a single value in an array', () => {
       const value = '_source';
-      const fields = parseMetaFields(value);
+      const fields = parseFields(value);
       expect(fields).toMatchInlineSnapshot(`
       Array [
         "_source",
@@ -40,7 +40,7 @@ describe('_fields_for_wildcard', () => {
 
     it('should return the array if already an array', () => {
       const value = ['_source', '_id'];
-      const fields = parseMetaFields(value);
+      const fields = parseFields(value);
       expect(fields).toMatchInlineSnapshot(`
       Array [
         "_source",

src/plugins/data_views/server/routes/fields_for.ts

@@ -23,14 +23,14 @@ import type { DataViewsServerPluginStart, DataViewsServerPluginStartDependencies
  * 2. A JSON-stringified array of field names
  * 3. A single field name (not comma-separated)
  * @returns an array of field names
- * @param metaFields
+ * @param fields
  */
-export const parseMetaFields = (metaFields: string | string[]): string[] => {
-  if (Array.isArray(metaFields)) return metaFields;
+export const parseFields = (fields: string | string[]): string[] => {
+  if (Array.isArray(fields)) return fields;
   try {
-    return JSON.parse(metaFields);
+    return JSON.parse(fields);
   } catch (e) {
-    if (!metaFields.includes(',')) return [metaFields];
+    if (!fields.includes(',')) return [fields];
     throw new Error(
       'metaFields should be an array of field names, a JSON-stringified array of field names, or a single field name'
     );
@@ -60,7 +60,7 @@ const validate: RouteValidatorFullConfig<{}, IQuery, IBody> = {
     rollup_index: schema.maybe(schema.string()),
     allow_no_index: schema.maybe(schema.boolean()),
     include_unmapped: schema.maybe(schema.boolean()),
-    fields: schema.maybe(schema.arrayOf(schema.string())),
+    fields: schema.maybe(schema.oneOf([schema.string(), schema.arrayOf(schema.string())])),
   }),
   // not available to get request
   body: schema.maybe(schema.object({ index_filter: schema.any() })),
@@ -81,8 +81,10 @@ const handler: RequestHandler<{}, IQuery, IBody> = async (context, request, resp
   const indexFilter = request.body?.index_filter;
 
   let parsedFields: string[] = [];
+  let parsedMetaFields: string[] = [];
   try {
-    parsedFields = parseMetaFields(metaFields);
+    parsedMetaFields = parseFields(metaFields);
+    parsedFields = parseFields(request.query.fields ?? []);
   } catch (error) {
     return response.badRequest();
   }
@@ -90,7 +92,7 @@ const handler: RequestHandler<{}, IQuery, IBody> = async (context, request, resp
   try {
     const { fields, indices } = await indexPatterns.getFieldsForWildcard({
       pattern,
-      metaFields: parsedFields,
+      metaFields: parsedMetaFields,
       type,
       rollupIndex,
       fieldCapsOptions: {
@@ -98,7 +100,7 @@ const handler: RequestHandler<{}, IQuery, IBody> = async (context, request, resp
         includeUnmapped,
       },
       indexFilter,
-      fields: request.query.fields,
+      ...(parsedFields.length > 0 ? { fields: parsedFields } : {}),
     });
 
     return response.ok({

test/api_integration/apis/data_views/fields_for_wildcard_route/params.js

@@ -22,15 +22,6 @@ export default function ({ getService }) {
     it('requires a pattern query param', () =>
       supertest.get('/api/index_patterns/_fields_for_wildcard').query({}).expect(400));
 
-    it('accepts a JSON formatted meta_fields query param', () =>
-      supertest
-        .get('/api/index_patterns/_fields_for_wildcard')
-        .query({
-          pattern: '*',
-          meta_fields: JSON.stringify(['meta']),
-        })
-        .expect(200));
-
     it('accepts include_unmapped param', () =>
       supertest
         .get('/api/index_patterns/_fields_for_wildcard')
@@ -40,33 +31,6 @@ export default function ({ getService }) {
         })
         .expect(200));
 
-    it('accepts meta_fields query param in string array', () =>
-      supertest
-        .get('/api/index_patterns/_fields_for_wildcard')
-        .query({
-          pattern: '*',
-          meta_fields: ['_id', 'meta'],
-        })
-        .expect(200));
-
-    it('accepts single meta_fields query param', () =>
-      supertest
-        .get('/api/index_patterns/_fields_for_wildcard')
-        .query({
-          pattern: '*',
-          meta_fields: ['_id'],
-        })
-        .expect(200));
-
-    it('rejects a comma-separated list of meta_fields', () =>
-      supertest
-        .get('/api/index_patterns/_fields_for_wildcard')
-        .query({
-          pattern: '*',
-          meta_fields: 'foo,bar',
-        })
-        .expect(400));
-
     it('rejects unexpected query params', () =>
       supertest
         .get('/api/index_patterns/_fields_for_wildcard')
@@ -75,5 +39,90 @@ export default function ({ getService }) {
           [randomness.word()]: randomness.word(),
         })
         .expect(400));
+
+    describe('fields', () => {
+      it('accepts a JSON formatted fields query param', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            fields: JSON.stringify(['baz']),
+          })
+          .expect(200));
+
+      it('accepts meta_fields query param in string array', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            fields: ['baz', 'foo'],
+          })
+          .expect(200));
+
+      it('accepts single array fields query param', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            fields: ['baz'],
+          })
+          .expect(200));
+
+      it('accepts single fields query param', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            fields: 'baz',
+          })
+          .expect(200));
+
+      it('rejects a comma-separated list of fields', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            fields: 'foo,bar',
+          })
+          .expect(400));
+    });
+
+    describe('meta_fields', () => {
+      it('accepts a JSON formatted meta_fields query param', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            meta_fields: JSON.stringify(['meta']),
+          })
+          .expect(200));
+
+      it('accepts meta_fields query param in string array', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            meta_fields: ['_id', 'meta'],
+          })
+          .expect(200));
+
+      it('accepts single meta_fields query param', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            meta_fields: ['_id'],
+          })
+          .expect(200));
+
+      it('rejects a comma-separated list of meta_fields', () =>
+        supertest
+          .get('/api/index_patterns/_fields_for_wildcard')
+          .query({
+            pattern: '*',
+            meta_fields: 'foo,bar',
+          })
+          .expect(400));
+    });
   });
 }