The output events are written to the Filebeat index because you've set index to use the name passed in the Beats metadata: index => "%{[@metadata][beat]}-%{[@metadata][version]}-%{+YYYY.MM.dd}". For a more detailed explanation, see the section about versioned beats indices in the docs. If you're planning to use the pre-built Beats dashboards, you generally do want to use this setting.
If you don't specify the index setting in the elasticsearch output stage, the name defaults to logstash-%{+YYYY.MM.dd}.