@Override
  public SystemStreamPartitionMapper getStreamPartitionMapper(Config config, MetricsRegistry metricsRegistry) {
    return new HashSystemStreamPartitionMapper();
  }
Instantiates the {@link HashSystemStreamPartitionMapper} based upon the provided
config and the metricsRegistry.
samza-core/src/main/java/org/apache/samza/container/grouper/stream/HashSystemStreamPartitionMapperFactory.java