[Discover] Switch from "at" to "charAt" for better browser support (e…

…lastic#156474)

From the community forum: users can't access Discover on older browsers.

```
TypeError: t.at is not a function
at http://xxx:5601/61109/bundles/plugin/data/kibana/data.plugin.js:1:312992
at Array.forEach ()
at n (http://xxx:5601/61109/bundles/plugin/data/kibana/data.plugin.js:1:312956)
at s (http://xxx:5601/61109/bundles/plugin/discover/kibana/discover.chunk.0.js:1:10138)
at http://xxx:5601/61109/bundles/plugin/discover/kibana/discover.chunk.0.js:1:10215
at Array.map ()
at a (http://xxx:5601/61109/bundles/plugin/discover/kibana/discover.chunk.0.js:1:10207)
at http://xxx:5601/61109/bundles/plugin/discover/kibana/discover.chunk.0.js:1:8442
at http://xxx:5601/61109/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:334:27724
at t.s._next (http://xxx:5601/61109/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:6:33456)
```

- https://discuss.elastic.co/t/update-to-8-7-0/329525
-
https://discuss.elastic.co/t/elastic-8-7-0-error-in-discover-tab/332329/2

## Summary

This PR replaces `String` `.at` usage with `.charAt` so it does not
require a polyfill for older browsers.

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

@@ -200,6 +200,7 @@ describe('tabify_docs', () => {
               _size: 12,
               _score: 77,
               _source: {},
+              invalid_meta: 'test',
             },
           ],
         },
@@ -216,7 +217,7 @@ describe('tabify_docs', () => {
             },
           },
         },
-        metaFields: ['_id', '_index', '_score', '_type', '_size'],
+        metaFields: ['_id', '_index', '_score', '_type', '_size', 'invalid_meta'],
         fieldFormats: fieldFormats as any,
       });
       const table = tabifyDocs(responsePlugin, dataView);

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

@@ -107,7 +107,8 @@ export function flattenHit(hit: Hit, indexPattern?: DataView, params?: TabifyDoc
 
   // Merge all valid meta fields into the flattened object
   indexPattern?.metaFields?.forEach((fieldName) => {
-    const isExcludedMetaField = EXCLUDED_META_FIELDS.includes(fieldName) || fieldName.at(0) !== '_';
+    const isExcludedMetaField =
+      EXCLUDED_META_FIELDS.includes(fieldName) || fieldName.charAt(0) !== '_';
     if (isExcludedMetaField) {
       return;
     }