Prometheus: Read feature toggles once when creating plugin instance (#102131)

* initialize an instance with feature toggles

* read grafana config only once

* use feature toggles directly

pkg/promlib/querydata/request_test.go

@@ -472,7 +472,7 @@ func setup() (*testContext, error) {
 		return nil, err
 	}
 
-	queryData, _ := querydata.New(httpClient, settings, log.New())
+	queryData, _ := querydata.New(httpClient, settings, log.New(), backend.FeatureToggles{})
 
 	return &testContext{
 		httpProvider: httpProvider,