Fixed the verbose log.warn in DFSUtil.addTransferRateMetric()

HDFS-16917 added a LOG.warn when passed duration is 0. The unit for duration is millis, and its very possible for a read to take less than a millisecond when considering local TCP connection. We are seeing this spam multiple times per millisecond. There's another report on the PR for HDFS-16917.

Please downgrade to debug or remove the log