Add method to list active users for a given flag

src/main/java/com/librato/rollout/RolloutClient.java

@@ -18,6 +18,12 @@ public interface RolloutClient {
      */
     boolean userFeatureActive(final String feature, long userId);
 
+    /**
+     * @param feature   Rollout feature
+     * @return list of active user ids
+     */
+    List<Long> activeUsers(final String feature);
+
     /**
      * @param feature Rollout feature
      * @return Percentage of given feature, or 0 if feature does not exist

src/main/java/com/librato/rollout/zk/RolloutZKClient.java

@@ -74,6 +74,12 @@ public boolean userFeatureActive(String feature, long userId, List<String> userG
         return false;
     }
 
+    @Override
+    public List<Long> activeUsers(String feature) {
+        final Entry entry = features.get().get(feature);
+        return entry.userIds;
+    }
+
     @Override
     public void start() throws Exception {
         if (!isStarted.compareAndSet(false, true)) {
@@ -156,7 +162,7 @@ public static Entry fromString(String s, String key) {
                     log.warn("Couldn't parse `{}` as a long, ignoring user id for key `{}`", id, key);
                 }
             }
-            return new Entry(percentage, userIds, groups);
+            return new Entry(percentage, Collections.unmodifiableList(userIds), Collections.unmodifiableList(groups));
         }
     }
 }