add `hostname key` to message from fluent-agent-lite

add_hostname.pl

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;
use JSON::XS;
use Data::MessagePack;

$| = 1;
my $mp = Data::MessagePack->new();
my $message_from = $ARGV[0];

while ( my $json = <STDIN> ) { 
  my $decode = eval { decode_json($json); };
  next if ($@);

  $decode->{hostname} = $message_from;

  print $mp->pack($decode);
}

td-agent.conf

<source>
  type forward
</source>

<match hoge.**>
  type forest
  subtype exec_filter
  remove_prefix hoge
  <template>
    command    /home/satoshi/perl5/perlbrew/perls/perl-5.16.1/bin/perl /etc/td-agent/scripts/add_hostname.pl ${tag}
    in_format  json
    out_format msgpack
    tag        foo.out
    flush_interval 1s
  </template>
</match>

<match foo.out>
  type file
  path /tmp/hoge.out
  flush_interval 1s
</match>