Fixes elastic#880 - Adds support for sending tags in gelf output

This adds tag support to the gelf output.

It adds a new option called ship_tags, set to true by default, when
sends all tags as a new field called _tags to GELF. Tags are converted
from an array to a comma separated list.

lib/logstash/outputs/gelf.rb

@@ -53,6 +53,10 @@ class LogStash::Outputs::Gelf < LogStash::Outputs::Base
   # messages.
   config :ship_metadata, :validate => :boolean, :default => true
 
+  # Ship tags within events. This will cause logstash to ship the tags of an
+  # event as the field _tags.
+  config :ship_tags, :validate => :boolean, :default => true
+
   # Ignore these fields when ship_metadata is set. Typically this lists the
   # fields used in dynamic values for GELF fields.
   config :ignore_metadata, :validate => :array, :default => [ "severity", "source_host", "source_path", "short_message" ]
@@ -156,6 +160,10 @@ def receive(event)
       end
     end
 
+    if @ship_tags
+      m["_tags"] = event.tags.join(', ')
+    end
+
     if @custom_fields
       @custom_fields.each do |field_name, field_value|
         m["_#{field_name}"] = field_value unless field_name == 'id'