Log message when async index creation completes

Add an additional log message when async index creation completes, so we
can better understand the progress without visibility into the primary
db.

lib/gitlab/database/async_indexes/index_creator.rb

@@ -23,6 +23,8 @@ module Gitlab
               set_statement_timeout do
                 connection.execute(async_index.definition)
               end
+
+              log_index_info('Finished creating async index')
             end
 
             async_index.destroy