Returns object that only collects some metrics.

        Returns an object which upon collect() will return
        only samples with the given names.

        Intended usage is:
            generate_latest(REGISTRY.restricted_registry(['a_timeseries']))

        Experimental.

def restricted_registry(self, names):
        """Returns object that only collects some metrics.

        Returns an object which upon collect() will return
        only samples with the given names.

        Intended usage is:
            generate_latest(REGISTRY.restricted_registry(['a_timeseries']))

        Experimental."""
        names = set(names)
        collectors = set()
        with self._lock:
            for name in names:
                if name in self._names_to_collectors:
                    collectors.add(self._names_to_collectors[name])
        metrics = []
        for collector in collectors:
            for metric in collector.collect():
                samples = [s for s in metric.samples if s[0] in names]
                if samples:
                    m = Metric(metric.name, metric.documentation, metric.type)
                    m.samples = samples
                    metrics.append(m)

        class RestrictedRegistry(object):
            def collect(self):
                return metrics

        return RestrictedRegistry()