Add line for number of cores to load graph

Backported from the node dashboard in the kubernetes-mixin.

Signed-off-by: beorn7 <beorn@grafana.com>

docs/node-mixin/dashboards/node.libsonnet

@@ -44,7 +44,8 @@ local gauge = promgrafonnet.gauge;
         )
         .addTarget(prometheus.target('node_load1{%(nodeExporterSelector)s, instance="$instance"}' % $._config, legendFormat='1m load average'))
         .addTarget(prometheus.target('node_load5{%(nodeExporterSelector)s, instance="$instance"}' % $._config, legendFormat='5m load average'))
-        .addTarget(prometheus.target('node_load15{%(nodeExporterSelector)s, instance="$instance"}' % $._config, legendFormat='15m load average'));
+        .addTarget(prometheus.target('node_load15{%(nodeExporterSelector)s, instance="$instance"}' % $._config, legendFormat='15m load average'))
+        .addTarget(prometheus.target('count(node_cpu_seconds_total{%(nodeExporterSelector)s, instance="$instance", mode="idle"})' % $._config, legendFormat='logical cores'));
 
       local memoryGraph =
         graphPanel.new(