filebeat:
  prospectors:
      paths:
        - /var/log/foo.log
      fields:
        type: foo
 output:
  logstash:

[...]


-------->


{
              "message" => "1444636537.881",
             "@version" => "1",
           "@timestamp" => "2015-11-02T23:43:39.881Z",
                 "type" => "log",
                "count" => 1,
               "fields" => {
               "type" => "foo"
    },
             "fileinfo" => {},
                 "line" => 14158,
               "offset" => 381315,
               "source" => "/var/log/foo",

[...]

}