Remove empty alert labels on ingest

The same behavior exists in prometheus. This is a
bit superfluous, but in the event people are using
old versions of prometheus or a different metric
gathering system, it's still valid to check.

api/api.go

@@ -496,6 +496,8 @@ func (api *API) insertAlerts(w http.ResponseWriter, r *http.Request, alerts ...*
 		validationErrs = &types.MultiError{}
 	)
 	for _, a := range alerts {
+		removeEmptyLabels(a.Labels)
+
 		if err := a.Validate(); err != nil {
 			validationErrs.Add(err)
 			numInvalidAlerts.Inc()
@@ -522,6 +524,14 @@ func (api *API) insertAlerts(w http.ResponseWriter, r *http.Request, alerts ...*
 	api.respond(w, nil)
 }
 
+func removeEmptyLabels(ls model.LabelSet) {
+	for k, v := range ls {
+		if string(v) == "" {
+			delete(ls, k)
+		}
+	}
+}
+
 func (api *API) setSilence(w http.ResponseWriter, r *http.Request) {
 	var sil types.Silence
 	if err := api.receive(r, &sil); err != nil {