add Citus specific metrics

[pashagolub]

• https://www.youtube.com/watch?v=f5f1XXYrx4I
• https://www.citusdata.com/blog/2023/05/12/tenant-monitoring-in-citus-and-postgres-with-citus-stat-tenants/
• https://www.citusdata.com/blog/2022/07/21/citus-stat-activity-views-for-distributed-postgres/