<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add(self, key, metric, **dims):
 """Adds custom metric instances to the registry with dimensions which are not created with their constructors default arguments """

        return super(MetricsRegistry, self).add(
            self.metadata.register(key, **dims), metric)