Add a collector to the registry.

def register(self, collector):
        """Add a collector to the registry."""
        with self._lock:
            names = self._get_names(collector)
            duplicates = set(self._names_to_collectors).intersection(names)
            if duplicates:
                raise ValueError(
                    'Duplicated timeseries in CollectorRegistry: {0}'.format(
                        duplicates))
            for name in names:
                self._names_to_collectors[name] = collector
            self._collector_to_names[collector] = names