How to handle Julian time in parsers

This is possible.
You need to use the date filter plugin. First, grok the string 202528209:12:13 into a temporary field, then use the date filter to parse it with the specific format pattern yyyyDDDHH:mm:ss

filter {
  grok {
    match => { "message" => "^%{DATA:log_timestamp} %{GREEDYDATA:rest_of_message}" }
  }
  if [log_timestamp] {
    date {
      match => [ "log_timestamp", "yyyyDDDHH:mm:ss" ]
      target => "event.idm.read_only_udm.metadata.event_timestamp" 
      on_error => "date_parse_failure"
    }
  }
}

You will find the date function listed under the section "Transform data using other functions”:
Parser syntax reference  |  Google Security Operations  |  Google Cloud Documentation

Hi,

I have already tried this yyyyDDDHH:mm:ss pattern but it is not working. “DDD” is not working for “day of year”.