@Override
  public void flush() {
    // Since the KV store will be flushed by task storage manager, only update metrics here
    instrument(metrics.numFlushes, metrics.flushNs, () -> { });
  }
Refer to {@link KeyValueStore#all()}

@return an iterator for all entries in this key-value store.
samza-kv/src/main/java/org/apache/samza/storage/kv/LocalTable.java