Filter mutate replace field value with value of another field

[magnusbaeck (Magnus Bäck)]

filter {
  mutate {
    update => { "host" => "%{[fields][tag_hostname]}" }
  }
 }

See https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#logstash-config-field-references and the following section.