func (t *tierMetrics) logFailure(tier string) {
	t.Lock()
	defer t.Unlock()

	stat := t.requestsCount[tier]
	stat.failure++
	t.requestsCount[tier] = stat
}

var (
	// {minio_node}_{tier}_{ttlb_seconds_distribution}
	tierTTLBMD = MetricDescription{
		Namespace: nodeMetricNamespace,
		Subsystem: tierSubsystem,
		Name:      ttlbDistribution,