Specify index, host, source, soucetype for the events.

index is a data repository in Splunk, please create the 4 indexes in Splunk: jenkins, jenkins_statistics, jenkins_console and jenkins_artifact

host is used to identify which Jenkins master is the source of the data. the metadata can be used in Splunk query language such as
index="jenkins" host="servername" sourcetype="json:jenkins"

See more details