code: def register(self, key, **kwargs):
          
          dimensions = dict((k, str(v)) for k, v in kwargs.items())
          composite_key = self._composite_name(key, dimensions)
          self._metadata[composite_key] = {
              'metric': key,
              'dimensions': dimensions
          }
          return composite_key
docs: Registers metadata for a metric and returns a composite key