title: trigger.Once() metadata needed
body: <p>Hi guys simple question for experienced guys. 
  I have a spark job reading files under a path.
  I wanted to use structured streaming even when the source is not really a stream but just a folder with a bunch of files in it.</p>
  
  <p>My question can I use trigger.Once() for this. And if yes how do I make trigger.Once recognizing new files as such. </p>
  
  <p>I tried it out on my laptop and the first run reads everything but when I start the job again files written in the mean time are not recognized and processed at all. </p>
  
  <p>my method looks like this: </p>
  
  <pre><code>def executeSql(spark:SparkSession):Unit ={
  
      val file = "home/hansherrlich/input_event/"
  
      val df  = spark.readStream.format("json").schema(getStruct).load("home/hansherrlich/some_event/")
  
      val out =    df.writeStream.trigger(Trigger.Once()).format("json").option("path","home/hansherrlich/some_event_processed/").start()
  
      out.processAllAvailable()
      out.stop()
      //out.awaitTermination()
      println("done writing")
  
  }
  </code></pre>