reset app gauges to omit old/removed apps

exporter.go

@@ -135,15 +135,15 @@ func (e *Exporter) scrapeApps(json *gabs.Container, ch chan<- prometheus.Metric)
  "avg_uptime": "taskStats.startedAfterLastScaling.stats.lifeTime.averageSeconds",
  }
 
+ name := "app_instances"
+ gauge, new := e.Gauges.Fetch(name, "Marathon app instance count", "app")
+ if new {
+ log.Infof("Added gauge %q\n", name)
+ }
+ gauge.Reset()
+
  for _, app := range elements {
  id := app.Path("id").Data().(string)
- name := "app_instances"
-
- gauge, new := e.Gauges.Fetch(name, "Marathon app instance count", "app")
- if new {
- log.Infof("Added gauge %q\n", name)
- }
-
  data := app.Path("instances").Data()
  count, ok := data.(float64)
  if !ok {