How to compute custom metrics using Elasticsearch + Kibana?
If you don't want an external application to do the analysis before indexing the documents in Elasticsearch, the best way I guess is to write a plugin that does it. You can write a plugin that implements a custom analyzer that does the sentiment analysis. Then in the mapping define the fields you want to run your analyzer on.
See examples of analysis plugins - https://github.com/barminator/elasticsearch-analysis-annotationhttps://github.com/yakaz/elasticsearch-analysis-combo/
To run the analysis on all existing documents you will need to reindex them after defining the correct mapping.