I submitted JSON to Logstash stdin input 
I forgot that part of the problem was to get the data into Elasticsearch as well. I have not tried to add fields dynamically to beats. Feels like the application fields should really be set by each app individually before Metricbeat even gets involved. But I have not used Metricbeat and I do not know if you can get it to pick up any metric fields or if there is a predefined schema and you have to stick with that.