service: fix for disabled internal metrics.

Update of the internal metrics for Grafana was
disabled by mistake when refactoring the code.

Fixes #15651

(cherry picked from commit 36788183d8)

pkg/cmd/grafana-server/server.go

@@ -29,6 +29,8 @@ import (
 	// self registering services
 	_ "github.com/grafana/grafana/pkg/extensions"
 	_ "github.com/grafana/grafana/pkg/infra/serverlock"
+
+	_ "github.com/grafana/grafana/pkg/infra/usagestats"
 	_ "github.com/grafana/grafana/pkg/metrics"
 	_ "github.com/grafana/grafana/pkg/plugins"
 	_ "github.com/grafana/grafana/pkg/services/alerting"