Loki: don't send empty vector selector to /labels API (#98547)

* fix: don't send empty vector selector to /labels API

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

@@ -461,6 +461,15 @@ describe('Language completion provider', () => {
         config.featureToggles.lokiLabelNamesQueryApi = lokiLabelNamesQueryApi;
       });
 
+      it('should exclude empty vector selector', async () => {
+        const datasourceWithLabels = setup({ foo: [], bar: [], __name__: [], __stream_shard__: [] });
+
+        const instance = new LanguageProvider(datasourceWithLabels);
+        instance.request = jest.fn();
+        await instance.fetchLabels({ streamSelector: '{}' });
+        expect(instance.request).toBeCalledWith('labels', { end: 1560163909000, start: 1560153109000 });
+      });
+
       it('should use series endpoint for request with stream selector', async () => {
         const datasourceWithLabels = setup({});
         datasourceWithLabels.languageProvider.request = jest.fn();

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

@@ -174,7 +174,7 @@ export default class LokiLanguageProvider extends LanguageProvider {
     const range = options?.timeRange ?? this.getDefaultTimeRange();
     const { start, end } = this.datasource.getTimeRangeParams(range);
     const params: Record<string, string | number> = { start, end };
-    if (options?.streamSelector) {
+    if (options?.streamSelector && options?.streamSelector !== EMPTY_SELECTOR) {
       params['query'] = options.streamSelector;
     }
     const res = await this.request(url, params);