Fix the computation of prometheus_sd_discovered_targets

prometheus_sd_discovered_targets is wrongly calculated when there are
multiple SD configurations in place. One discovery manager can have
multiple groups coming from multiple service discoveries.

When multiple service discovery configs are used, we do not compute the
metric correctly, and instead just set the metric to one of the service
discoveries.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

discovery/manager.go

@@ -279,15 +279,17 @@ func (m *Manager) allGroups() map[string][]*targetgroup.Group {
 	defer m.mtx.RUnlock()
 
 	tSets := map[string][]*targetgroup.Group{}
+	n := map[string]int{}
 	for pkey, tsets := range m.targets {
-		var n int
 		for _, tg := range tsets {
 			// Even if the target group 'tg' is empty we still need to send it to the 'Scrape manager'
 			// to signal that it needs to stop all scrape loops for this target set.
 			tSets[pkey.setName] = append(tSets[pkey.setName], tg)
-			n += len(tg.Targets)
+			n[pkey.setName] += len(tg.Targets)
 		}
-		discoveredTargets.WithLabelValues(m.name, pkey.setName).Set(float64(n))
+	}
+	for setName, v := range n {
+		discoveredTargets.WithLabelValues(m.name, setName).Set(float64(v))
 	}
 	return tSets
 }