Add Metrics class with serialization and documentation

Added a new Metrics class to represent key performance metrics of a benchmark test run.

- The Metrics class encapsulates various data points like difficulty, success rate, attempted status, success percentage, cost, and runtime.
- Included serialization and deserialization methods for converting between Metrics objects and JSON.
- Added comprehensive documentation to describe the purpose, properties, and methods of the Metrics class.

frontend/lib/models/benchmark/metrics.dart

@@ -0,0 +1,67 @@
+/// `Metrics` holds key performance metrics related to a benchmark test run.
+///
+/// The class encapsulates various data points like difficulty, success rate,
+/// whether the task was attempted, the percentage of success, and other performance metrics.
+class Metrics {
+  /// The perceived difficulty level of the test, usually represented as a string.
+  final String difficulty;
+
+  /// A boolean indicating whether the test was successful.
+  final bool success;
+
+  /// A boolean indicating whether the test was attempted.
+  final bool attempted;
+
+  /// The percentage of success in the test, represented as a double.
+  final double successPercentage;
+
+  /// The cost metric, can be any type depending on what is being measured (time, resources, etc.).
+  /// It is dynamic to allow for various types.
+  final dynamic cost;
+
+  /// The total runtime of the test, represented as a string.
+  final String runTime;
+
+  /// Constructs a new `Metrics` instance.
+  ///
+  /// [difficulty]: The perceived difficulty level of the test.
+  /// [success]: A boolean indicating the success status of the test.
+  /// [attempted]: A boolean indicating if the test was attempted.
+  /// [successPercentage]: The success rate as a percentage.
+  /// [cost]: The cost metric for the test.
+  /// [runTime]: The total runtime of the test.
+  Metrics({
+    required this.difficulty,
+    required this.success,
+    required this.attempted,
+    required this.successPercentage,
+    required this.cost,
+    required this.runTime,
+  });
+
+  /// Creates a `Metrics` instance from a map.
+  ///
+  /// [json]: A map containing key-value pairs corresponding to `Metrics` fields.
+  ///
+  /// Returns a new `Metrics` populated with values from the map.
+  factory Metrics.fromJson(Map<String, dynamic> json) => Metrics(
+        difficulty: json['difficulty'],
+        success: json['success'],
+        attempted: json['attempted'],
+        successPercentage: json['success_percentage'].toDouble(),
+        cost: json['cost'],
+        runTime: json['run_time'],
+      );
+
+  /// Converts the `Metrics` instance to a map.
+  ///
+  /// Returns a map containing key-value pairs corresponding to `Metrics` fields.
+  Map<String, dynamic> toJson() => {
+        'difficulty': difficulty,
+        'success': success,
+        'attempted': attempted,
+        'success_percentage': successPercentage,
+        'cost': cost,
+        'run_time': runTime,
+      };
+}