[ONOS-6806] Add getAvailableDeviceCount method to DeviceStore

Change-Id: I3f9e0bbd502e2a592da98436685fa2a192436ef8
diff --git a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
index 00dacc2..7f1539d 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
@@ -276,6 +276,11 @@
     }
 
     @Override
+    public int getAvailableDeviceCount() {
+        return availableDevices.size();
+    }
+
+    @Override
     public Iterable<Device> getDevices() {
         return Collections.unmodifiableCollection(devices.values());
     }