How to Create Email Alerts for Tracking Untracked Hosts on Kibana?

Question

I have a dynamic file called known_hosts present on remote server that contains a list of all known hosts, which is updated daily using a cron job. The data from each host is sent to Logstash every 5 minutes using another cron job. Now, I want to compare the list of hosts present on Kibana with the hosts listed in the known_hosts file. I need to send an email alert listing the hosts present in known_hosts but not on Kibana.

I know I can use watchers on Kibana for this purpose but how should I compare the values present on host_list and output of Kibana query.

{
  "trigger": {
    "schedule": {
      "interval": "5m" // should it be every 5 mins or per day , as known_host file updates each 24 hrs but data is send to kibana from hosts every 5 mins
    }
  },
  "input": {
    "search": {
      "request": {
        "body": {
          "size": 0,
          "query": {
            // my query
          }
        },
        "indices": [
          "my_index_*"
        ]
      }
    }
  },
  "condition": {
    "compare": {
      // how should I compare the output of input query with known_host file.
    }
  },
  "actions": {
    // email alert
  }
}

How can I achieve this? Any suggestions or code examples would be greatly appreciated. Thank you!