mg.descriptorMap = make(map[MetricName]MetricDescriptor, len(mg.Descriptors))
	for _, desc := range mg.Descriptors {
		mg.descriptorMap[desc.Name] = desc
	}
}

// SetCache is a helper to initialize MetricsGroup. It sets the cache object.
func (mg *MetricsGroup) SetCache(c *metricsCache) {
	mg.cache = c