How to access the value in the logstash metadata

Elastic Stack Logstash
2

Something like this?

input {
  generator {
    lines => ['message1']
    count => 1
    add_field => { "[@metadata][key]" => "test" }
  }
}

filter {
  if [@metadata][key] == "test" {
    mutate {
      add_tag => ["found"]
    }
  }
}

output {
  stdout { codec => rubydebug { metadata => true }}
}