HADOOP-13855. Fix a couple of the s3a statistic names to be consistent with the rest. Contributed by Steve Loughran

(cherry picked from commit 6b6bc4705d2b608e810ae7d1912eac49bfd4c6ca)

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/Statistic.java

@@ -92,12 +92,12 @@ public enum Statistic {
       "Count of times the TCP stream was aborted"),
   STREAM_BACKWARD_SEEK_OPERATIONS("stream_backward_seek_operations",
       "Number of executed seek operations which went backwards in a stream"),
-  STREAM_CLOSED("streamClosed", "Count of times the TCP stream was closed"),
+  STREAM_CLOSED("stream_closed", "Count of times the TCP stream was closed"),
   STREAM_CLOSE_OPERATIONS("stream_close_operations",
       "Total count of times an attempt to close a data stream was made"),
   STREAM_FORWARD_SEEK_OPERATIONS("stream_forward_seek_operations",
       "Number of executed seek operations which went forward in a stream"),
-  STREAM_OPENED("streamOpened",
+  STREAM_OPENED("stream_opened",
       "Total count of times an input stream to object store was opened"),
   STREAM_READ_EXCEPTIONS("stream_read_exceptions",
       "Number of seek operations invoked on input streams"),