Change default value for sorting dropdown in settings (tensorflow#7051)

## Motivation for features / changes
The tooltip is now limited to 5 items, so the default sorting should
prioritize the most relevant data. Setting Nearest Pixel as the default
ensures the tooltip shows the items closest to the cursor, reducing
clutter and improving readability.

## Technical description of changes
Updated the dropdown settings view component to use Nearest Pixel as the
default sorting option. This affects the initial tooltip ordering logic
so the first 5 displayed items are the ones closest to the cursor
position.

Author: cdavalos7

tensorboard/webapp/metrics/metrics_integration_test.ts

@@ -24,6 +24,7 @@ import {MetricsDashboardContainer} from './views/metrics_container';
 describe('metrics integration test', () => {
   const ByCss = {
     SCALARS_SMOOTHING_INPUT: By.css('.scalars-smoothing input'),
+    TOOLTIP_SORT_DROPDOWN: By.css('.tooltip-sort tb-dropdown'),
   };
 
   beforeEach(() => {
@@ -44,6 +45,13 @@ describe('metrics integration test', () => {
 
       const input = fixture.debugElement.query(ByCss.SCALARS_SMOOTHING_INPUT);
       expect(input.nativeElement.value).toBe('0.6');
+
+      const tooltipSortDropdown = fixture.debugElement.query(
+        ByCss.TOOLTIP_SORT_DROPDOWN
+      );
+      expect(tooltipSortDropdown.componentInstance.value).toBe(
+        METRICS_SETTINGS_DEFAULT.tooltipSort
+      );
     });
 
     it('permits overriding default settings with DI', async () => {

tensorboard/webapp/metrics/store/metrics_types.ts

@@ -278,7 +278,7 @@ export interface State {
 
 export const METRICS_SETTINGS_DEFAULT: MetricsSettings = {
   cardMinWidth: null,
-  tooltipSort: TooltipSort.ALPHABETICAL,
+  tooltipSort: TooltipSort.NEAREST,
   ignoreOutliers: true,
   xAxisType: XAxisType.STEP,
   hideEmptyCards: true,