ProcessBookkeepingService encode logged items for ES ingestion

ES ingestion won't be able to handle an array of different types and
thus the log entry will be blocked. As such we can't find these log
statements in Kibana at the moment. Changing to just a string will allow
it to be ingested properly.

ee/app/services/elastic/process_bookkeeping_service.rb

@@ -20,7 +20,7 @@ module Elastic
           min = (max - items.size) + 1
 
           (min..max).zip(items).each_slice(1000) do |group|
-            logger.debug(message: 'track_items', count: group.count, items: group)
+            logger.debug(message: 'track_items', count: group.count, tracked_items_encoded: group.to_json)
 
             redis.zadd(REDIS_SET_KEY, group)
           end