// Helper function to register a collector and return a gatherer for it.
	mustRegister := func(c ...prometheus.Collector) prometheus.Gatherer {
		subRegistry := prometheus.NewRegistry()
		for _, col := range c {
			subRegistry.MustRegister(col)
		}
		r.MustRegister(subRegistry)
		return subRegistry
	}

	// Register all collectors and create gatherers for them.