default Map<String, SystemStreamMetadata> getSystemStreamPartitionCounts(Set<String> streamNames, long cacheTTL) {
    return getSystemStreamMetadata(streamNames);
  }
Get partitions counts only. Should be more efficient then getSystemStreamMetadata, but if not implemented
revert to getSystemStreamMetadata.
@param streamNames set of streams to query.
@param cacheTTL cacheTTL to use if caching the values.
@return A map from stream name to SystemStreamMetadata for each stream
       requested in the parameter set.
samza-api/src/main/java/org/apache/samza/system/SystemAdmin.java