Pass through current agent Consul datacenter name

Instead of only filling __meta_consul_dc when datacenter is set in
consul_sd_config this change fills the label based on what the agent
reports it's current data center is, if datacenter isn't manually set,
otherwise it uses whatever datacenter was set to.

retrieval/discovery/consul.go

@@ -54,6 +54,7 @@ const (
 type ConsulDiscovery struct {
 	client           *consul.Client
 	clientConf       *consul.Config
+	clientDatacenter string
 	tagSeparator     string
 	scrapedServices  map[string]struct{}
 
@@ -95,6 +96,17 @@ func NewConsulDiscovery(conf *config.ConsulSDConfig) *ConsulDiscovery {
 		scrapedServices: map[string]struct{}{},
 		services:        map[string]*consulService{},
 	}
+	// If the datacenter isn't set in the clientConf, let's get it from the local Consul agent
+	// (Consul default is to use local node's datacenter if one isn't given for a query).
+	if clientConf.Datacenter == "" {
+		info, err := client.Agent().Self()
+		if err != nil {
+			panic(fmt.Errorf("discovery.NewConsulDiscovery: %s", err))
+		}
+		cd.clientDatacenter = info["Config"]["Datacenter"].(string)
+	} else {
+		cd.clientDatacenter = clientConf.Datacenter
+	}
 	for _, name := range conf.Services {
 		cd.scrapedServices[name] = struct{}{}
 	}
@@ -214,7 +226,7 @@ func (cd *ConsulDiscovery) watchServices(update chan<- *consulService, done <-ch
 			}
 			srv.tgroup.Labels = clientmodel.LabelSet{
 				ConsulServiceLabel: clientmodel.LabelValue(name),
-				ConsulDCLabel:      clientmodel.LabelValue(cd.clientConf.Datacenter),
+				ConsulDCLabel:      clientmodel.LabelValue(cd.clientDatacenter),
 			}
 			update <- srv
 		}