Provide max_file_count option\n to define the maximum number of files to upload at once

src/main/java/org/embulk/input/filename/FilenameFileInputPlugin.java

@@ -112,6 +112,10 @@ public class FilenameFileInputPlugin implements FileInputPlugin
         @ConfigDefault("null")
         Optional<Integer> getFileSize();
 
+        @Config("max_file_count")
+        @ConfigDefault("null")
+        Optional<Integer> getMaxFileCount();
+        
         @Config("follow_symlinks")
         @ConfigDefault("false")
         boolean getFollowSymlinks();
@@ -120,6 +124,7 @@ public class FilenameFileInputPlugin implements FileInputPlugin
         @ConfigDefault("false")
         boolean getIgnoreLastFile();
 
+
         List<String> getFiles();
         void setFiles(List<String> files);
 
@@ -137,7 +142,15 @@ public class FilenameFileInputPlugin implements FileInputPlugin
         PluginTask task = config.loadConfig(PluginTask.class);
 
         // list files recursively
-        List<String> files = listFiles(task);
+        List<String> files = new ArrayList<String>(listFiles(task));
+        Collections.sort(files);
+        
+        // only process <= max_file_count number of files
+        final Integer maxFileCount = task.getMaxFileCount().orNull();
+        if (maxFileCount != null && files.size() > maxFileCount) {
+            files = files.subList(0, maxFileCount);
+        }
+        
         log.info("Loading files {}", files);
         task.setFiles(files);