how to get logstash to output key=value pairs from JSON? For example, from below JSON

apache-access_jsonlog-postmaster.dev.returnpath.net.log

{"@timestamp":"2015-02-05T12:52:21-0700","@version":"1","tags":["apache","access"],"docker":{"container_id":"CONTAINER_ID","host":"HOST"},"product":"PRODUCT","container_id":"1","bytes":"0","client_ip":"127.0.0.1","client_port":"53053","duration_micros":"219","http_method":"OPTIONS","keepalive":"-","method":"OPTIONS","pid":"117","protocol":"HTTP/1.0","referer":"-","request":"*","response_code":"200","response_size":"-","server_ip":"127.0.0.1","service_port":"80","session_id":"-","site":"-","status":"200","url_path":"*","url_query":"","user":"-","user-agent":"Apache/2.4.6 (CentOS) PHP/5.4.16 (internal dummy connection)","poma_user_id":"-","vhost":"-","X-Forwarded-For":"-"}

logstash.conf

input {
  file {
    codec => "json"
    path => "/tmp/log/apache-access_jsonlog-postmaster.dev.returnpath.net.log"
  }
}
filter{
  json{
    source => "message"
    add_field => {
      "foo" => "bar"
      "KITTEH" => "DOGGEH"
    }
  }
}
output {
  stdout {
    codec => rubydebug
  }
}

logstash.stdout

{
         "@timestamp" => "2015-02-05T13:57:03.000-07:00",
           "@version" => "1",
               "tags" => [
        [0] "apache",
        [1] "access"
    ],
             "docker" => {
        "container_id" => "CONTAINER_ID",
                "host" => "HOST"
    },
            "product" => "PRODUCT",
       "container_id" => "1",
              "bytes" => "0",
          "client_ip" => "127.0.0.1",
        "client_port" => "53179",
    "duration_micros" => "203",
        "http_method" => "OPTIONS",
          "keepalive" => "-",
             "method" => "OPTIONS",
                "pid" => "134",
           "protocol" => "HTTP/1.0",
            "referer" => "-",
            "request" => "*",
      "response_code" => "200",
      "response_size" => "-",
          "server_ip" => "127.0.0.1",
       "service_port" => "80",
         "session_id" => "-",
               "site" => "-",
             "status" => "200",
           "url_path" => "*",
          "url_query" => "",
               "user" => "-",
         "user-agent" => "Apache/2.4.6 (CentOS) PHP/5.4.16 (internal dummy connection)",
       "poma_user_id" => "-",
              "vhost" => "-",
    "X-Forwarded-For" => "-",
               "host" => "jcrotty.dev.returnpath.net",
               "path" => "/tmp/log/apache-access_jsonlog-postmaster.dev.returnpath.net.log"
}