metrics: use "standard" appearance of button-toggle

In OSS, mat-button-toggle's appearance is "standard" but this is not the
case internally. To make the appearance consistent, we are now
explicitly specifying the type of appearance we want.

tensorboard/webapp/metrics/views/main_view/main_view_component.ng.html

@@ -16,7 +16,7 @@
 -->
 <div class="toolbar">
   <metrics-tag-filter></metrics-tag-filter>
-  <mat-button-toggle-group class="filter-view" multiple>
+  <mat-button-toggle-group class="filter-view" multiple appearance="standard">
     <mat-button-toggle
       [checked]="filteredPluginTypes.size === 0"
       (click)="onPluginTypeAllToggled.emit()"