This task tracks fields conflicts found by elasticsearch/logstash when ingesting logs. Such errors happen when the same field name (i.e. a key in a json structured log) appears with values of different types in the same index.
For example application 1 sending "foo": "bar" and application 2 sending "foo": 1 will cause an error.
More context on this problem: T180051: Reduce the number of fields declared in elasticsearch by logstash and related.