isExactlyOneSet := (mg.loader == nil) != (mg.bucketLoader == nil)
	if !isExactlyOneSet {
		panic("Exactly one Loader function must be set")
	}

	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.