FSNamesystemLock metrics can be inaccurate due to millisecond precision

Currently the FSNamesystemLock metrics created in HDFS-10872 track the lock hold time using Timer.monotonicNow(), which has millisecond-level precision. However, many of these operations hold the lock for less than a millisecond, making these metrics inaccurate. We should instead use System.nanoTime() for higher accuracy.