Slow FileSystem.Statistics counters implementation

While profiling 1TB TeraGen job on Hadoop 2.8.2 cluster (Google Dataproc, 2 workers, GCS connector) I saw that FileSystem.Statistics code paths Wall time is 5.58% and CPU time is 26.5% of total execution time.

After switching FileSystem.Statistics implementation to LongAdder, consumed Wall time decreased to 0.006% and CPU time to 0.104% of total execution time.

Total job runtime decreased from 66 mins to 61 mins.

These results are not conclusive, because I didn't benchmark multiple times to average results, but regardless of performance gains switching to LongAdder simplifies code and reduces its complexity.