MetricsSourceAdapter#updateAttrCache may throw NPE due to NULL lastRecs

  private int updateAttrCache() {
    LOG.debug("Updating attr cache...");
    int recNo = 0;
    int numMetrics = 0;
    for (MetricsRecordImpl record : lastRecs) {
      for (MetricsTag t : record.tags()) {
        setAttrCacheTag(t, recNo);
        ++numMetrics;
      }
      for (AbstractMetric m : record.metrics()) {
        setAttrCacheMetric(m, recNo);
        ++numMetrics;
      }
      ++recNo;
    }
    LOG.debug("Done. # tags & metrics="+ numMetrics);
    return numMetrics;
  }