Add decide method for feature gate creation

apps/mail/lib/flags.ts

@@ -12,4 +12,7 @@ export const createFeatureGate = (key: string) => flag<boolean, StatsigUser>({
     key,
     adapter: statsigAdapter.featureGate((gate) => gate.value, { exposureLogging: true }),
     identify,
+    decide() {
+        return Math.random() > 0.1;
+    },
 });