code: def add_gauge(self, gauge):
          
          if gauge is None:
              raise ValueError
          name = gauge.descriptor.name
          with self._gauges_lock:
              if name in self.gauges:
                  raise ValueError(
                      'Another gauge named "{}" is already registered'
                      .format(name))
              self.gauges[name] = gauge
docs: Add `gauge` to the registry.
  
          Raises a `ValueError` if another gauge with the same name already
          exists in the registry.
  
          :type gauge: class:`LongGauge`, class:`DoubleGauge`,
              :class:`opencensus.metrics.export.cumulative.LongCumulative`,
              :class:`opencensus.metrics.export.cumulative.DoubleCumulative`,
              :class:`DerivedLongGauge`, :class:`DerivedDoubleGauge`
              :class:`opencensus.metrics.export.cumulative.DerivedLongCumulative`,
              or
              :class:`opencensus.metrics.export.cumulative.DerivedDoubleCumulative`
          :param gauge: The gauge to add to the registry.