elk stack - how to generate new fields in logstash -

i need generate new fields (loglevel) using logstash,finally displaying in kibana.

1. how extract log , make pattern using grok filter log.
2. how create field of loglevel using logstash configuration.

i found this page helpful when setting log4net filter. based on logs like, you'll end (copied page):

filter {   if [type] == "log4net" {     grok {       match => [ "message", "(?m)%{loglevel:level} %{timestamp_iso8601:sourcetimestamp} %{data:logger} \[%{number:threadid}\]  \[%{iporhost:temphost}\] %{greedydata:tempmessage}" ]     }     mutate {         replace => [ "message" , "%{tempmessage}" ]         replace => [ "host" , "%{temphost}" ]         remove_field => [ "tempmessage" ]         remove_field => [ "temphost" ]     }   } }