code: public LabelCache labels(String monitorId)
      {
          LabelCache cache = labels.get(monitorId);
          if(cache == null)
              labels.put(monitorId, cache = new LabelCache(monitorId));
          return cache;
      }
docs: Returns the cache of labels for the given monitor, creating one if it doesn't exist .
  @param monitorId The id of the monitor for the cache of labels
  @return The cache of labels for the given monitor