Prometheus: Fix running of exemplar queries for non-histogram metrics (#42749)

* Prometheus: Fix running of exemplar queries for non-histogram mestrics

* Fix test

* Refactor based on review suggestions

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

@@ -1667,6 +1667,35 @@ describe('PrometheusDatasource', () => {
       templateSrvStub.replace = jest.fn((a: string) => a);
     });
   });
+
+  it('should give back 1 exemplar target when multiple queries with exemplar enabled and same metric', () => {
+    const targetA: PromQuery = {
+      refId: 'A',
+      expr: 'histogram_quantile(0.95, sum(rate(tns_request_duration_seconds_bucket[5m])) by (le))',
+      exemplar: true,
+    };
+    const targetB: PromQuery = {
+      refId: 'B',
+      expr: 'histogram_quantile(0.5, sum(rate(tns_request_duration_seconds_bucket[5m])) by (le))',
+      exemplar: true,
+    };
+
+    ds.languageProvider = {
+      histogramMetrics: ['tns_request_duration_seconds_bucket'],
+    } as any;
+
+    const request = ({
+      targets: [targetA, targetB],
+      interval: '1s',
+      panelId: '',
+    } as any) as DataQueryRequest<PromQuery>;
+
+    const Aexemplars = ds.shouldRunExemplarQuery(targetA, request);
+    const BExpemplars = ds.shouldRunExemplarQuery(targetB, request);
+
+    expect(Aexemplars).toBe(true);
+    expect(BExpemplars).toBe(false);
+  });
 });
 
 describe('PrometheusDatasource for POST', () => {

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

@@ -300,21 +300,19 @@ export class PrometheusDatasource extends DataSourceWithBackend<PromQuery, PromO
     };
   };
 
-  shouldRunExemplarQuery(target: PromQuery): boolean {
-    /* We want to run exemplar query only for histogram metrics:
-    1. If we haven't processd histogram metrics yet, we need to check if expr includes "_bucket" which means that it is probably histogram metric (can rarely lead to false positive).
-    2. If we have processed histogram metrics, check if it is part of query expr.
-    */
+  shouldRunExemplarQuery(target: PromQuery, request: DataQueryRequest<PromQuery>): boolean {
     if (target.exemplar) {
-      const histogramMetrics = this.languageProvider.histogramMetrics;
+      // We check all already processed targets and only create exemplar target for not used metric names
+      const metricName = this.languageProvider.histogramMetrics.find((m) => target.expr.includes(m));
+      // Remove targets that weren't processed yet (in targets array they are after current target)
+      const currentTargetIdx = request.targets.findIndex((t) => t.refId === target.refId);
+      const targets = request.targets.slice(0, currentTargetIdx);
 
-      if (histogramMetrics.length > 0) {
-        return !!histogramMetrics.find((metric) => target.expr.includes(metric));
-      } else {
-        return target.expr.includes('_bucket');
+      if (!metricName || (metricName && !targets.some((t) => t.expr.includes(metricName)))) {
+        return true;
       }
+      return false;
     }
-
     return false;
   }
 
@@ -322,7 +320,7 @@ export class PrometheusDatasource extends DataSourceWithBackend<PromQuery, PromO
     const processedTarget = {
       ...target,
       queryType: PromQueryType.timeSeriesQuery,
-      exemplar: this.shouldRunExemplarQuery(target),
+      exemplar: this.shouldRunExemplarQuery(target, request),
       requestId: request.panelId + target.refId,
       // We need to pass utcOffsetSec to backend to calculate aligned range
       utcOffsetSec: this.timeSrv.timeRange().to.utcOffset() * 60,