@Override
  public Optional<Path> checkpoint(CheckpointId id) {
    // No checkpoint being persisted. State restores from Changelog.
    return Optional.empty();
  }
@param metrics A metrics instance to publish key-value store related statistics
samza-kv-inmemory/src/main/java/org/apache/samza/storage/kv/inmemory/InMemoryKeyValueStore.java