Prometheus: Fix for prometheus legend formats for instant time series queries (#24407)

* Prometheus: Fix for prometheus legend formats for instance time series queries

* correct old test

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

@@ -736,7 +736,7 @@ describe('PrometheusDatasource', () => {
       const frame = toDataFrame(results.data[0]);
       expect(results.data.length).toBe(1);
       expect(frame.name).toBe('test{job="testjob"}');
-      expect(getFieldTitle(frame.fields[1])).toBe('Value');
+      expect(getFieldTitle(frame.fields[1])).toBe('test{job="testjob"}');
     });
   });
 

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

@@ -116,6 +116,27 @@ describe('Prometheus Result Transformer', () => {
     });
   });
 
+  describe('When resultFormat is time series and instant = true', () => {
+    const response = {
+      status: 'success',
+      data: {
+        resultType: 'vector',
+        result: [
+          {
+            metric: { __name__: 'test', job: 'testjob' },
+            value: [1443454528, '3846'],
+          },
+        ],
+      },
+    };
+
+    it('should return time series', () => {
+      const timeSeries = ctx.resultTransformer.transform({ data: response }, {});
+      expect(timeSeries[0].target).toBe('test{job="testjob"}');
+      expect(timeSeries[0].title).toBe('test{job="testjob"}');
+    });
+  });
+
   describe('When resultFormat is heatmap', () => {
     const response = {
       status: 'success',

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

@@ -143,7 +143,7 @@ export class ResultTransformer {
     const dps = [];
     const { name, labels } = this.createLabelInfo(md.metric, options);
     dps.push([parseFloat(md.value[1]), md.value[0] * 1000]);
-    return { target: name, datapoints: dps, tags: labels, refId: options.refId, meta: options.meta };
+    return { target: name, title: name, datapoints: dps, tags: labels, refId: options.refId, meta: options.meta };
   }
 
   createLabelInfo(labels: { [key: string]: string }, options: any): { name?: string; labels: Labels; title?: string } {