kube-prometheus: Don't drop non-pod metrics

These metrics can still be valuable to troubleshoot components running
in cgroups on the host, outside the scope of Kubernetes pods.

jsonnet/kube-prometheus/prometheus/prometheus.libsonnet

@@ -283,13 +283,6 @@ local k = import 'ksonnet/ksonnet.beta.3/k.libsonnet';
               },
               bearerTokenFile: '/var/run/secrets/kubernetes.io/serviceaccount/token',
               metricRelabelings: [
-                // Drop container_* metrics with no image.
-                {
-                  sourceLabels: ['__name__', 'image'],
-                  regex: 'container_([a-z_]+);',
-                  action: 'drop',
-                },
-
                 // Drop a bunch of metrics which are disabled but still sent, see
                 // https://github.com/google/cadvisor/issues/1925.
                 {