Map<Short, Map<Short, Long>> getContainerPhysicalStorageUsage() {
    return Collections.unmodifiableMap(containerPhysicalStorageUsage);
  }
Given a reference time in ms return the corresponding valid data size per container map by aggregating all buckets
whose end time is less than or equal to the reference time.
@param referenceTimeInMs the reference time in ms until which deletes and expiration are relevant.
@return a {@link Pair} whose first element is the end time of the last bucket that was aggregated and whose second
element is the requested valid data size per container {@link Map}.
ambry-store/src/main/java/com/github/ambry/store/ScanResults.java