Truncate large string values

Resolves #26.

lib/fluent/plugin/out_gelf.rb

@@ -9,6 +9,8 @@ class GELFOutput < BufferedOutput
   config_param :host, :string, :default => nil
   config_param :port, :integer, :default => 12201
   config_param :protocol, :string, :default => 'udp'
+  # Default max_bytes is set to avoid Elasticsearch indexing errors.
+  config_param :max_bytes, :integer, :default => 32000
 
   def initialize
     super
@@ -54,6 +56,9 @@ def format(tag, time, record)
     gelfentry = { :timestamp => timestamp, :_tag => tag }
 
     record.each_pair do |k,v|
+      # Truncate values longer than configured maximum
+      v = v.bytesize > @max_bytes ? "#{v.byteslice(0, @max_bytes - 3)}..." : v
+
       case k
       when 'version' then
         gelfentry[:_version] = v