Expose Quota Metrics to Prometheus

In 3.7, Quota limit can be enforced and the quota related stats are captured in the StatsTrack. From the "listquota" CLI command, we can the quota limit and usage info.

As an addition to that, we would like to collect the quota metrics and expose them to the Prometheus for the following:

1. Monitoring per namespace (Chroot) quota usage via the Grafana dashboard
2. Creating alert based on the quota levels (e.g. 90% used)