Add ScrollMetrics.extentTotal for completeness (#126607)

When implementing scrollbars, I found that it would be useful and idiomatic to be able to do `m.extentInside / m.extentTotal` to get the scrollbar thumb size.

packages/flutter/lib/src/widgets/scroll_metrics.dart

@@ -115,11 +115,12 @@ mixin ScrollMetrics {
   /// This is the content above the content described by [extentInside].
   double get extentBefore => math.max(pixels - minScrollExtent, 0.0);
 
-  /// The quantity of content conceptually "inside" the viewport in the scrollable.
+  /// The quantity of content conceptually "inside" the viewport in the
+  /// scrollable (including empty space if the total amount of content is less
+  /// than the [viewportDimension]).
   ///
-  /// The value is typically the height of the viewport when [outOfRange] is false.
-  /// It could be less if there is less content visible than the size of the
-  /// viewport, such as when overscrolling.
+  /// The value is typically the extent of the viewport ([viewportDimension])
+  /// when [outOfRange] is false. It can be less when overscrolling.
   ///
   /// The value is always non-negative, and less than or equal to [viewportDimension].
   double get extentInside {
@@ -135,6 +136,12 @@ mixin ScrollMetrics {
   /// This is the content below the content described by [extentInside].
   double get extentAfter => math.max(maxScrollExtent - pixels, 0.0);
 
+  /// The total quantity of content available.
+  ///
+  /// This is the sum of [extentBefore], [extentInside], and [extentAfter], modulo
+  /// any rounding errors.
+  double get extentTotal => maxScrollExtent - minScrollExtent + viewportDimension;
+
   /// The [FlutterView.devicePixelRatio] of the view that the [Scrollable]
   /// associated with this metrics object is drawn into.
   double get devicePixelRatio;

packages/flutter/test/widgets/scroll_notification_test.dart

@@ -31,6 +31,7 @@ void main() {
     expect(event.metrics.extentBefore, 0.0);
     expect(event.metrics.extentInside, 600.0);
     expect(event.metrics.extentAfter, 400.0);
+    expect(event.metrics.extentTotal, 1000.0);
 
     events.clear();
     final TestGesture gesture = await tester.startGesture(const Offset(100.0, 100.0));
@@ -53,6 +54,7 @@ void main() {
     expect(event.metrics.extentBefore, 10.0);
     expect(event.metrics.extentInside, 590.0);
     expect(event.metrics.extentAfter, 0.0);
+    expect(event.metrics.extentTotal, 600.0);
 
     events.clear();
     // The content dimensions does not change.