Make metadata abstract.

Author: kinyoklion

packages/common_client/lib/src/hooks/hook.dart

@@ -78,11 +78,22 @@ final class TrackSeriesContext {
 ///
 /// Default implementations are provided for each stage and an implementer
 /// should override at least one of the stage methods.
-base class Hook {
-  final HookMetadata metadata;
+///
+/// All implementations must implement the metadata getter.
+abstract base class Hook {
+  /// Metadata associated with this hook.
+  ///
+  /// Hook implementations must implement this property.
+  /// ```dart
+  /// final _metadata = HookMetadata(name: 'MyHookName');
+  ///
+  /// @override
+  /// HookMetadata get metadata => _metadata;
+  /// ```
+  HookMetadata get metadata;
 
   /// Construct a new hook instance.
-  Hook(this.metadata);
+  Hook();
 
   /// This method is called during the execution of a variation method before
   /// the flag value has been determined. The method is executed synchronously.

packages/common_client/test/hooks/hook_runner_test.dart

@@ -16,16 +16,21 @@ final class TestHook extends Hook {
   final String? errorMessage;
   final List<UnmodifiableMapView<String, LDValue>> receivedBeforeData = [];
   final List<UnmodifiableMapView<String, LDValue>> receivedAfterData = [];
+  final HookMetadata _metadata;
 
   // For tracking execution order across multiple hooks
   static final List<String> globalExecutionOrder = [];
 
+  @override
+  HookMetadata get metadata => _metadata;
+
   TestHook({
     required String name,
     this.dataToReturn = const {},
     this.shouldThrow = false,
     this.errorMessage,
-  }) : super(HookMetadata(name: name));
+  })  : _metadata = HookMetadata(name: name),
+        super();
 
   void clearGlobalOrder() {
     globalExecutionOrder.clear();

packages/common_client/test/hooks/ld_common_client_hooks_test.dart

@@ -11,8 +11,14 @@ final class TestHook extends Hook {
   final List<TrackSeriesContext> trackContexts = [];
   final List<IdentifyResult> identifyResults = [];
   final List<LDEvaluationDetail<LDValue>> evaluationResults = [];
+  final HookMetadata _metadata;
 
-  TestHook(String name) : super(HookMetadata(name: name));
+  @override
+  HookMetadata get metadata => _metadata;
+
+  TestHook(String name)
+      : _metadata = HookMetadata(name: name),
+        super();
 
   @override
   UnmodifiableMapView<String, LDValue> beforeEvaluation(