Fix relabaling collision when using exported label

When using both a label and the suffix+label in the
relabel config. It's possible that Prometheus remove
the suffx+label for no obvious reason. It's due to a
collision when merging labels from target and from
the sample.

Signed-off-by: Geoffrey Beausire <g.beausire@criteo.com>

scrape/scrape.go

@@ -451,9 +451,9 @@ func mutateSampleLabels(lset labels.Labels, target *Target, honor bool, rc []*re
 		for _, l := range target.Labels() {
 			// existingValue will be empty if l.Name doesn't exist.
 			existingValue := lset.Get(l.Name)
-			// Because setting a label with an empty value is a no-op,
-			// this will only create the prefixed label if necessary.
-			lb.Set(model.ExportedLabelPrefix+l.Name, existingValue)
+			if existingValue != "" {
+				lb.Set(model.ExportedLabelPrefix+l.Name, existingValue)
+			}
 			// It is now safe to set the target label.
 			lb.Set(l.Name, l.Value)
 		}

scrape/scrape_test.go

@@ -935,6 +935,15 @@ func TestScrapeLoopAppend(t *testing.T) {
 			discoveryLabels: []string{"n", "2"},
 			expLset:         labels.FromStrings("__name__", "metric", "exported_n", "1", "n", "2"),
 			expValue:        0,
+		}, {
+			// When "honor_labels" is not set
+			// exported label from discovery don't get overwritten
+			title:           "Label name collision",
+			honorLabels:     false,
+			scrapeLabels:    `metric 0`,
+			discoveryLabels: []string{"n", "2", "exported_n", "2"},
+			expLset:         labels.FromStrings("__name__", "metric", "n", "2", "exported_n", "2"),
+			expValue:        0,
 		}, {
 			// Labels with no value need to be removed as these should not be ingested.
 			title:           "Delete Empty labels",