Refactor truncate_message filter

We extract some internal emthods to make the truncation flow clearer,
i.e. first we run the selectors, then we truncate the message, then we
insert the ellipsis message (if required).

lib/rackstash/filter/truncate_message.rb

@@ -97,36 +97,54 @@ module Rackstash
         end
         return event if messages.size <= 1
 
+        truncate(messages)
+
+        event
+      end
+
+      private
+
+      def truncate(messages)
         overall_size = overall_size_of(messages)
         ellipsis = nil
+
         until overall_size <= @max_size || messages.size <= 1
-          case @cut
-          when :top
-            msg = messages.shift
-          when :middle
-            msg = messages.delete_at(messages.size / 2)
-          when :bottom
-            msg = messages.pop
-          end
+          msg =
+            case @cut
+            when :top
+              messages.shift
+            when :middle
+              messages.delete_at(messages.size / 2)
+            when :bottom
+              messages.pop
+            end
+
           overall_size -= msg.size
+
           unless ellipsis || @ellipsis.nil?
             ellipsis = Rackstash::Message.new(@ellipsis)
             overall_size += ellipsis.size
           end
         end
 
-        if ellipsis
-          case @cut
-          when :top then messages.unshift(ellipsis)
-          when :middle then messages.insert((messages.size + 1) / 2, ellipsis)
-          when :bottom then messages.push(ellipsis)
-          end
-        end
+        # Insert the ellipsis message if we have truncated any messages
+        insert_into(messages, ellipsis) if ellipsis
 
-        event
+        messages
       end
 
-      private
+      def insert_into(messages, msg)
+        case @cut
+        when :top
+          messages.unshift(msg)
+        when :middle
+          messages.insert((messages.size + 1) / 2, msg)
+        when :bottom
+          messages.push(msg)
+        end
+
+        messages
+      end
 
       def overall_size_of(messages)
         messages.inject(0) { |sum, msg| sum + msg.size }