code: public Double getFileStoreMetric(String fileStoreNameName, ID metricToCollect) {
  
          Map<String, OSFileStore> cache = getFileStores();
          OSFileStore fileStore = cache.get(fileStoreNameName);
          if (fileStore == null) {
              return null;
          }
  
          if (PlatformMetricType.FILE_STORE_TOTAL_SPACE.getMetricTypeId().equals(metricToCollect)) {
              return Double.valueOf(fileStore.getTotalSpace());
          } else if (PlatformMetricType.FILE_STORE_USABLE_SPACE.getMetricTypeId().equals(metricToCollect)) {
              return Double.valueOf(fileStore.getUsableSpace());
          } else {
              throw new UnsupportedOperationException("Invalid file store metric to collect: " + metricToCollect);
          }
      }
docs: Returns the given metric's value, or null if there is no file store with the given name.
  
  @param fileStoreNameName name of file store
  @param metricToCollect the metric to collect
  @return the value of the metric, or null if there is no file store with the given name