prom: fix sorting in `fetchLabels` (#102318)

* fix(prom): return label keys from prometheus language providers

* fix: only sort once

* fix: lose reference

packages/grafana-prometheus/src/language_provider.ts

@@ -251,7 +251,7 @@ export default class PromQlLanguageProvider extends LanguageProvider {
     const res = await this.request(url, [], searchParams, this.getDefaultCacheHeaders());
     if (Array.isArray(res)) {
       this.labelKeys = res.slice().sort();
-      return res.slice().sort();
+      return [...this.labelKeys];
     }
 
     return [];