don't (re) create a string from a numeric timestamp, just return the string we get

src/main/java/org/elasticsearch/action/index/IndexRequest.java

@@ -655,7 +655,7 @@ public class IndexRequest extends ShardReplicationOperationRequest {
 
         // generate timestamp if not provided, we always have one post this stage...
         if (timestamp == null) {
-            timestamp = String.valueOf(System.currentTimeMillis());
+            timestamp = Long.toString(System.currentTimeMillis());
         }
     }
 

src/main/java/org/elasticsearch/cluster/metadata/MappingMetaData.java

@@ -160,7 +160,9 @@ public class MappingMetaData {
         public static String parseStringTimestamp(String timestampAsString, FormatDateTimeFormatter dateTimeFormatter) throws TimestampParsingException {
             long ts;
             try {
+                // if we manage to parse it, its a millisecond timestamp, just return the string as is
                 ts = Long.parseLong(timestampAsString);
+                return timestampAsString;
             } catch (NumberFormatException e) {
                 try {
                     ts = dateTimeFormatter.parser().parseMillis(timestampAsString);
@@ -168,7 +170,7 @@ public class MappingMetaData {
                     throw new TimestampParsingException(timestampAsString);
                 }
             }
-            return String.valueOf(ts);
+            return Long.toString(ts);
         }