Add runit exporter

[discordianfish]

It exports one metric with a label for the service name and labels for the services status:

  {
     "metric" : {
        "value" : [
           {
              "value" : 1091,
              "labels" : {
                 "want" : "down",
                 "name" : "test",
                 "normally_up" : "yes",
                 "state" : "down"
              }
           },
           {
              "value" : 8,
              "labels" : {
                 "want" : "up",
                 "name" : "test",
                 "normally_up" : "yes",
                 "state" : "up"
              }
           }
        ],
        "type" : "gauge"
     },
     "docstring" : "node_exporter: status of runit service.",
     "baseLabels" : {
        "name" : "node_service_status"
     }
  }

But maybe we should have one metric per service? Not sure about that.

[matttproud]

Cruft.