type rewriteFunc func(mf *dto.MetricFamily) (*dto.MetricFamily, error)

func (m *monitorGatherer) Gather() ([]*dto.MetricFamily, error) {
	etcdMetrics, err := scrapeMetrics()
	if err != nil {
		return nil, err
	}
	exported, err := m.rewriteExportedMetrics(etcdMetrics)
	if err != nil {
		return nil, err
	}
	custom, err := customMetricRegistry.Gather()
	if err != nil {
		return nil, err
	}