Add namespace metadata to deleted pods metric

[linki]

Adds the namespace as a label to the pods_deleted metric. The result looks like this:

Fixes #150

# HELP chaoskube_pods_deleted_total The total number of pods deleted
# TYPE chaoskube_pods_deleted_total counter
chaoskube_pods_deleted_total{namespace="baz"} 1
chaoskube_pods_deleted_total{namespace="api"} 1
chaoskube_pods_deleted_total{namespace="cassandra"} 1
chaoskube_pods_deleted_total{namespace="foobar"} 1
chaoskube_pods_deleted_total{namespace="default"} 27
chaoskube_pods_deleted_total{namespace="kube-system"} 31
chaoskube_pods_deleted_total{namespace="github"} 1
chaoskube_pods_deleted_total{namespace="myspace"} 1
chaoskube_pods_deleted_total{namespace="other"} 4