refactor: Cherry-pick DH-20363: Expose paddingMap reset method on IrisGridMetricCalculator (#2619)

Expose a method allowing to reset private `cachedPaddingMaps` in
subclasses. This method is needed for the Pivot plugin to reset header
group widths when the filter bar is expanded or collapsed.

Author: vbabich

packages/iris-grid/src/IrisGridMetricCalculator.ts

@@ -232,11 +232,19 @@ export class IrisGridMetricCalculator extends GridMetricCalculator {
       !deepEqual(headerGroupNames, this.cachedHeaderGroupNames)
     ) {
       this.resetCalculatedColumnWidths();
-      this.cachedPaddingMaps.clear();
     }
     this.cachedHeaderGroupNames = headerGroupNames;
   }
 
+  resetCalculatedHeaderGroupWidths(): void {
+    this.cachedPaddingMaps.clear();
+  }
+
+  resetCalculatedColumnWidths(): void {
+    super.resetCalculatedColumnWidths();
+    this.resetCalculatedHeaderGroupWidths();
+  }
+
   getGridY(state: IrisGridMetricState): number {
     // The state here seems to be a GridMetricState with stateOverrides passed from IrisGrid in the props,
     // not guaranteed to be IrisGridMetricState