public long remainingTimeNs(long startNs) {
    return timeoutNs - (System.nanoTime() - startNs);
  }
Returns the nanos, if any, for which the client should wait
@param error The exception from the server
@param tries The current retry count
@return The time, in nanos, to pause. If empty then pausing would exceed our timeout, so we
        should throw now
hbase-client/src/main/java/org/apache/hadoop/hbase/client/backoff/HBaseServerExceptionPauseManager.java