Report hot reload statistics. (#26039)

* Report hot reload statistic

* Lints

* Introduce custom dimensions identifiers to track hot reload stats

packages/flutter_tools/lib/src/run_hot.dart

@@ -592,10 +592,10 @@ class HotRunner extends ResidentRunner {
   }
 
   Future<OperationResult> _reloadSources({ bool pause = false, String reason }) async {
-    final Map<String, String> analyticsParameters =
-      reason == null
-        ? null
-        : <String, String>{ kEventReloadReasonParameterName: reason };
+    final Map<String, String> analyticsParameters = <String, String> {};
+    if (reason != null) {
+      analyticsParameters[kEventReloadReasonParameterName] = reason;
+    }
     for (FlutterDevice device in flutterDevices) {
       for (FlutterView view in device.views) {
         if (view.uiIsolate == null)
@@ -663,6 +663,18 @@ class HotRunner extends ResidentRunner {
           flutterUsage.sendEvent('hot', 'reload-reject');
           return OperationResult(1, 'Reload rejected');
         } else {
+          // Collect stats that help understand scale of update for this hot reload request.
+          // For example, [syncedLibraryCount]/[finalLibraryCount] indicates how
+          // many libraries were affected by the hot reload request.
+          // Relation of [invalidatedSourcesCount] to [syncedLibraryCount] should help
+          // understand sync/transfer "overhead" of updating this number of source files.
+          final Map<String, dynamic> details = reloadReport['details'];
+          analyticsParameters[kEventReloadFinalLibraryCount] = "${details['finalLibraryCount']}";
+          analyticsParameters[kEventReloadSyncedLibraryCount] = "${details['receivedLibraryCount']}";
+          analyticsParameters[kEventReloadSyncedClassesCount] = "${details['receivedClassesCount']}";
+          analyticsParameters[kEventReloadSyncedProceduresCount] = "${details['receivedProceduresCount']}";
+          analyticsParameters[kEventReloadSyncedBytes] = '${updatedDevFS.syncedBytes}';
+          analyticsParameters[kEventReloadInvalidatedSourcesCount] = '${updatedDevFS.invalidatedSourcesCount}';
           flutterUsage.sendEvent('hot', 'reload', parameters: analyticsParameters);
           final int loadedLibraryCount = reloadReport['details']['loadedLibraryCount'];
           final int finalLibraryCount = reloadReport['details']['finalLibraryCount'];

packages/flutter_tools/lib/src/usage.dart

@@ -17,6 +17,12 @@ import 'version.dart';
 
 const String _kFlutterUA = 'UA-67589403-6';
 const String kEventReloadReasonParameterName = 'cd5';
+const String kEventReloadFinalLibraryCount = 'cd6';
+const String kEventReloadSyncedLibraryCount = 'cd7';
+const String kEventReloadSyncedClassesCount = 'cd8';
+const String kEventReloadSyncedProceduresCount = 'cd9';
+const String kEventReloadSyncedBytes = 'cd10';
+const String kEventReloadInvalidatedSourcesCount = 'cd11';
 
 Usage get flutterUsage => Usage.instance;