make query-frontend service grpc lb friendly (#8025)

**What this PR does / why we need it**:
In PR #7966, I made changes to Query Frontend service. Part of it was
creating a new service.
To make the service properly load balance grpc requests, we need to use
`grpclbServiceFor`, but since we do not use grpc in QF, I went with the
usual method, i.e. I used `serviceFor`. Thinking more about it, I think
there is no harm in keeping it grpc LB friendly, and it would save us
from any possible bugs if we make QF accept queries via grpc.

production/ksonnet/loki/query-frontend.libsonnet

@@ -60,5 +60,5 @@ local k = import 'ksonnet-util/kausal.libsonnet';
     service.mixin.metadata.withName('query-frontend-headless'),
 
   query_frontend_service:
-    k.util.serviceFor($.query_frontend_deployment, $._config.service_ignored_labels),
+    $.util.grpclbServiceFor($.query_frontend_deployment),
 }