Map<String, Map<String, Long>> getBackupFileContent(String filename) throws IOException {
      if (!backupFiles.contains(filename)) {
        return null;
      }
      Path filePath = backupDirPath.resolve(filename);
      Map<String, Map<String, Long>> content = objectMapper.readValue(filePath.toFile(), typeReference);
      return content;
    }
Return the container usage in map from given {@code filename}.
@param filename The filename to load container usage.
@return A map that represents container usage.
@throws IOException
ambry-quota/src/main/java/com/github/ambry/quota/storage/MySqlStorageUsageRefresher.java