Implement new getImpactMetrics method for fake unleash implementation

Fake unleash implementation is only used in unleash demo app. The new
method is not relevant, therefore just returns `null`.

Author: denniseffing

demo-apps/chaos-monkey-demo-app-unleash-toggles/src/main/java/com/example/chaos/monkey/toggledemo/UserAwareFakeUnleash.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021-2025 the original author or authors.
+ * Copyright 2021-2026 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,7 @@
 import io.getunleash.Unleash;
 import io.getunleash.UnleashContext;
 import io.getunleash.UnleashContextProvider;
+import io.getunleash.impactmetrics.MetricsAPI;
 import io.getunleash.variant.Variant;
 
 import java.util.function.BiPredicate;
@@ -87,4 +88,9 @@ public Variant getVariant(String s, Variant variant) {
     public MoreOperations more() {
         return null;
     }
+
+    @Override
+    public MetricsAPI getImpactMetrics() {
+        return null;
+    }
 }