public void updatePartitionMapping(Map<TaskName, Integer> prevChangelogEntries,
      Map<TaskName, Integer> newChangelogEntries) {
    Map<TaskName, Integer> combinedEntries = new HashMap<>(newChangelogEntries);
    combinedEntries.putAll(prevChangelogEntries);
    writePartitionMapping(combinedEntries);
  }
Merges the previous and the new taskName to changelog partition mapping.
Writes the merged taskName to partition mapping to {@link MetadataStore}.
@param prevChangelogEntries The previous map of taskName to changelog partition.
@param newChangelogEntries The new map of taskName to changelog partition.
samza-core/src/main/java/org/apache/samza/storage/ChangelogStreamManager.java