fix: label_values w/ empty filters in prom datasource variable query (#78242)

* fix: handle empty filters in prometheus datasource variable query function `label_values`

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

@@ -104,6 +104,30 @@ describe('PrometheusMetricFindQuery', () => {
       });
     });
 
+    const emptyFilters = ['{}', '{   }', ' {   }  ', '   {}  '];
+
+    emptyFilters.forEach((emptyFilter) => {
+      const queryString = `label_values(${emptyFilter}, resource)`;
+      it(`Empty filter, query, ${queryString} should just generate label search query`, async () => {
+        const query = setupMetricFindQuery({
+          query: queryString,
+          response: {
+            data: ['value1', 'value2', 'value3'],
+          },
+        });
+        const results = await query.process(raw);
+
+        expect(results).toHaveLength(3);
+        expect(fetchMock).toHaveBeenCalledTimes(1);
+        expect(fetchMock).toHaveBeenCalledWith({
+          method: 'GET',
+          url: `/api/datasources/uid/ABCDEF/resources/api/v1/label/resource/values?start=${raw.from.unix()}&end=${raw.to.unix()}`,
+          hideFromInspector: true,
+          headers: {},
+        });
+      });
+    });
+
     // <LegacyPrometheus>
     it('label_values(metric, resource) should generate series query with correct time', async () => {
       const query = setupMetricFindQuery({

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

@@ -48,10 +48,13 @@ export default class PrometheusMetricFindQuery {
 
     const labelValuesQuery = this.query.match(labelValuesRegex);
     if (labelValuesQuery) {
-      if (labelValuesQuery[1]) {
-        return this.labelValuesQuery(labelValuesQuery[2], labelValuesQuery[1]);
+      const filter = labelValuesQuery[1];
+      const label = labelValuesQuery[2];
+      if (isFilterDefined(filter)) {
+        return this.labelValuesQuery(label, filter);
       } else {
-        return this.labelValuesQuery(labelValuesQuery[2]);
+        // Exclude the filter part of the expression because it is blank or empty
+        return this.labelValuesQuery(label);
       }
     }
 
@@ -192,3 +195,8 @@ export default class PrometheusMetricFindQuery {
     });
   }
 }
+
+function isFilterDefined(filter: string) {
+  // We consider blank strings or the empty filter {} as an undefined filter
+  return filter && filter.split(' ').join('') !== '{}';
+}