gnore unused cpus in Prometheus metrics

Only include cpu's in the Prometheus metrics endpoint that were used.

In recent kernels the cpuacct.statcpus behavior has changed to include
all possible cpu's. This can results in a high number of stale metrics
in the Prometheus endpoint.

metrics/prometheus.go

@@ -130,10 +130,12 @@ func NewPrometheusCollector(i infoProvider, f ContainerLabelsFunc) *PrometheusCo
 				getValues: func(s *info.ContainerStats) metricValues {
 					values := make(metricValues, 0, len(s.Cpu.Usage.PerCpu))
 					for i, value := range s.Cpu.Usage.PerCpu {
-						values = append(values, metricValue{
-							value:  float64(value) / float64(time.Second),
-							labels: []string{fmt.Sprintf("cpu%02d", i)},
-						})
+						if value > 0 {
+							values = append(values, metricValue{
+								value:  float64(value) / float64(time.Second),
+								labels: []string{fmt.Sprintf("cpu%02d", i)},
+							})
+						}
 					}
 					return values
 				},