Add freeze to temporal string key

Author: repeatedly

lib/fluent/plugin/filter_concat.rb

@@ -130,7 +130,7 @@ def filter_stream(tag, es)
           end
         end
         if @mode == :partial_metadata
-          unless record.key?("partial_message")
+          unless record.key?("partial_message".freeze)
             new_es.add(time, record)
             next
           end
@@ -146,10 +146,10 @@ def filter_stream(tag, es)
                 merged_record.delete(@partial_key) unless @keep_partial_key
               when :partial_metadata
                 unless @keep_partial_metadata
-                  merged_record.delete("partial_message")
-                  merged_record.delete("partial_id")
-                  merged_record.delete("partial_ordinal")
-                  merged_record.delete("partial_last")
+                  merged_record.delete("partial_message".freeze)
+                  merged_record.delete("partial_id".freeze)
+                  merged_record.delete("partial_ordinal".freeze)
+                  merged_record.delete("partial_last".freeze)
                 end
               end
               new_es.add(time, merged_record)