datatrails: only fetch metrics metadata when using the language provider, avoid redundant fetch of metric names (#87067)

* fix: only fetch metrics metadata, not metric names

public/app/features/trails/helpers/MetricDatasourceHelper.ts

@@ -36,7 +36,7 @@ export class MetricDatasourceHelper {
 
     if (ds.languageProvider instanceof PromQlLanguageProvider) {
       if (!ds.languageProvider.metricsMetadata) {
-        await ds.languageProvider.start();
+        await ds.languageProvider.loadMetricsMetadata();
       }
 
       return ds.languageProvider.metricsMetadata!;