code: public static HashCode sign(MetricValue value) {
      Hasher h = Hashing.md5().newHasher();
      return putMetricValue(h, value).hash();
    }
docs: Obtains the {@code HashCode} for the contents of {@code value}.
  
  @param value a {@code MetricValue} to be signed
  @return the {@code HashCode} corresponding to {@code value}