[Field Formatters] Fix field format convert() return value (#211342)

## Summary

It was [pointed
out](#209824 (comment))
during a recent review that the field format `convert()` value does some
odd casting. It looks like this probably isn't necessary since
`converter` should never be undefined, and we can just return the result
directly.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [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
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Elastic Machine <[email protected]>

src/platform/packages/shared/kbn-discover-contextual-components/src/data_types/logs/components/summary_column/summary_column.test.tsx

@@ -22,6 +22,7 @@ import { sharePluginMock } from '@kbn/share-plugin/public/mocks';
 import { coreMock as corePluginMock } from '@kbn/core/public/mocks';
 import { DataTableRecord, buildDataTableRecord } from '@kbn/discover-utils';
 import { dataViewMock } from '@kbn/discover-utils/src/__mocks__/data_view';
+import type { IFieldFormatsRegistry } from '@kbn/field-formats-plugin/common';
 
 jest.mock('@elastic/eui', () => ({
   ...jest.requireActual('@elastic/eui'),
@@ -46,7 +47,15 @@ const getSummaryProps = (
   isDetails: false,
   row: record,
   dataView: dataViewMock,
-  fieldFormats: fieldFormatsMock,
+  fieldFormats: {
+    ...fieldFormatsMock,
+    getDefaultInstance: jest
+      .fn()
+      .mockImplementation((...params: Parameters<IFieldFormatsRegistry['getDefaultInstance']>) => ({
+        ...fieldFormatsMock.getDefaultInstance(...params),
+        convert: jest.fn().mockImplementation((t: string) => String(t)),
+      })),
+  },
   setCellProps: () => {},
   closePopover: () => {},
   density: DataGridDensity.COMPACT,

src/platform/packages/shared/kbn-discover-contextual-components/src/data_types/logs/components/summary_column/utils.tsx

@@ -181,9 +181,7 @@ export const createResourceFields = ({
     return {
       name,
       rawValue: resourceDoc[name],
-      // TODO: formatFieldValue doesn't actually return a string in certain circumstances, change
-      // this line below once it does.
-      value: typeof value === 'string' ? value : `${value}`,
+      value,
       ResourceBadge: getResourceBadgeComponent(name, core, share),
       Icon: getResourceBadgeIcon(name, resourceDoc),
     };

src/platform/plugins/shared/discover/public/components/data_types/logs/service_name_cell.tsx

@@ -58,9 +58,7 @@ export const getServiceNameCell =
         onFilter={actions.addFilter}
         icon={getIcon}
         rawValue={serviceNameValue}
-        // TODO: formatFieldValue doesn't actually return a string in certain circumstances, change
-        // this line below once it does.
-        value={typeof value === 'string' ? value : `${value}`}
+        value={value}
         property={serviceNameField}
         core={core}
         share={share}

src/platform/plugins/shared/field_formats/common/field_format.ts

@@ -117,14 +117,7 @@ export abstract class FieldFormat {
     contentType: FieldFormatsContentType = DEFAULT_CONTEXT_TYPE,
     options?: HtmlContextTypeOptions | TextContextTypeOptions
   ): string {
-    const converter = this.getConverterFor(contentType);
-
-    if (converter) {
-      return converter.call(this, value, options);
-    }
-
-    // TODO: should be "return `${value}`;", but might be a breaking change
-    return value as string;
+    return this.getConverterFor(contentType).call(this, value, options);
   }
 
   /**
@@ -140,7 +133,7 @@ export abstract class FieldFormat {
       this.convertObject = this.setupContentType();
     }
 
-    return this.convertObject[contentType];
+    return this.convertObject[contentType] ?? this.convertObject.text;
   }
 
   /**