jfjoch_broker: Allow to query connected writers

receiver/JFJochReceiverService.cpp

@@ -337,3 +337,7 @@ JFJochReceiverService &JFJochReceiverService::Indexing(const IndexingSettings &i
         throw JFJochException(JFJochExceptionCategory::WrongDAQState,
                               "Cannot change indexing settings during data collection");
 }
+
+uint64_t JFJochReceiverService::GetConnectedWriters() const {
+    return image_pusher.GetConnectedWriters();
+}