lib/promrelabel: make a copy of label with new name for action: labelmap in the same way as Prometheus does

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/812

lib/promrelabel/relabel.go

@@ -206,8 +206,9 @@ func applyRelabelConfig(labels []prompbmarshal.Label, labelsOffset int, prc *Par
 			}
 			value := relabelBufPool.Get()
 			value.B = prc.Regex.ExpandString(value.B[:0], prc.Replacement, label.Name, match)
-			label.Name = string(value.B)
+			labelName := string(value.B)
 			relabelBufPool.Put(value)
+			labels = setLabelValue(labels, labelsOffset, labelName, label.Value)
 		}
 		return labels
 	case "labelmap_all":

lib/promrelabel/relabel_test.go

@@ -575,6 +575,10 @@ func TestApplyRelabelConfigs(t *testing.T) {
 				Name:  "foo",
 				Value: "yyy",
 			},
+			{
+				Name:  "foobar",
+				Value: "aaa",
+			},
 		})
 	})
 	t.Run("labelmap_all", func(t *testing.T) {

lib/promscrape/config_test.go

@@ -920,6 +920,10 @@ scrape_configs:
 					Name:  "prefix:url",
 					Value: "http://foo.bar:1234/metrics",
 				},
+				{
+					Name:  "url",
+					Value: "http://foo.bar:1234/metrics",
+				},
 			},
 			AuthConfig:      &promauth.Config{},
 			jobNameOriginal: "foo",