It is not part of that logstash configuration you need to create a mapping before you start loading data from your database.
The command I showed you you can run it in Kibana -> Dev Tools
The proper Overall Steps are (you went directly to step 2)
- Create a Mapping / Index Template (this is the schema for your data in Elasticsearch I linked the docs and gave you a sample)
- Configure and Run Logstash
- Investigate Data in Discover
You could clean up the data and try the convert filter in logstash to set timedifference to type float but you will need to delete the index and try again.
But in my opinion the best approach is to create a mapping