@zhi_asher, you are correct that there is an error in the example execution statement. There should be only two closing braces and I'll get that fixed.
As for your plugin, I see no problems in what you've posted so far. At this point, I'd probably need all of the entire source to determine where the problem is. One thing I would suggest is testing with Logstash 7.2 if you're able. That version contains the GA release for Java plugins and it automatically generates all the Ruby "glue" code so there is a much reduced likelihood of naming or versioning mismatches.