Prometheus: Render missing labels in legend formats as an empty string (#22355)

* Empty value for unknown Prometheus label (grafana#16041)

* Empty value for unknown Prometheus label (grafana#16041)

* fix linter

pkg/tsdb/prometheus/prometheus.go

@@ -140,8 +140,7 @@ func formatLegend(metric model.Metric, query *PrometheusQuery) string {
 		if val, exists := metric[model.LabelName(labelName)]; exists {
 			return []byte(val)
 		}
-
-		return in
+		return []byte{}
 	})
 
 	return string(result)

pkg/tsdb/prometheus/prometheus_test.go

@@ -28,7 +28,7 @@ func TestPrometheus(t *testing.T) {
 				LegendFormat: "legend {{app}} {{ device }} {{broken}}",
 			}
 
-			So(formatLegend(metric, query), ShouldEqual, "legend backend mobile {{broken}}")
+			So(formatLegend(metric, query), ShouldEqual, "legend backend mobile ")
 		})
 
 		Convey("build full serie name", func() {

public/app/plugins/datasource/prometheus/result_transformer.ts

@@ -166,7 +166,7 @@ export class ResultTransformer {
       if (aliasData[g1]) {
         return aliasData[g1];
       }
-      return g1;
+      return '';
     });
   }