Loki Language Provider: Add missing interpolation to fetchLabelsByLabelsEndpoint (#114608)

* Plugins: Implement bug fix for loki label selectors w/ variable interpolation

* Chore: Add test to ensure result is interpolated

---------

Co-authored-by: Zoltán Bedi <zoltan.bedi@gmail.com>

public/app/plugins/datasource/loki/LanguageProvider.test.ts

@@ -560,6 +560,23 @@ describe('Language completion provider', () => {
           start: 1560153109000,
         });
       });
+
+      it('should interpolate variables in stream selector', async () => {
+        const datasource = setup({});
+        jest.spyOn(datasource, 'getTimeRangeParams').mockReturnValue({ start: 0, end: 1 });
+        jest
+          .spyOn(datasource, 'interpolateString')
+          .mockImplementation((string: string) => string.replace(/\$test_var/g, 'age'));
+
+        const languageProvider = new LanguageProvider(datasource);
+        languageProvider.request = jest.fn().mockResolvedValue([]);
+        await languageProvider.fetchLabels({ streamSelector: '{age="new", $test_var="new"}' });
+        expect(languageProvider.request).toHaveBeenCalledWith('labels', {
+          end: 1,
+          query: '{age="new", age="new"}',
+          start: 0,
+        });
+      });
     });
 
     it('should filter internal labels', async () => {

public/app/plugins/datasource/loki/LanguageProvider.ts

@@ -175,7 +175,8 @@ export default class LokiLanguageProvider extends LanguageProvider {
     const { start, end } = this.datasource.getTimeRangeParams(range);
     const params: Record<string, string | number> = { start, end };
     if (options?.streamSelector && options?.streamSelector !== EMPTY_SELECTOR) {
-      params['query'] = options.streamSelector;
+      const interpolatedStreamSelector = this.datasource.interpolateString(options.streamSelector);
+      params['query'] = interpolatedStreamSelector;
     }
     const res = await this.request(url, params);
     if (Array.isArray(res)) {