Storage stats research

The cluster show command needs to collect "storage statistics" from the cluster. Such as:

• Number of caches
• Number of partitions
• Number of tables
• Number of databases
• Number of schemas
• Number of distribution zones

As far as I know, there is no such API. The goals of this ticket are

• investigate possible ways of how we could collect such stats
• create follow-up tickets
• design REST API

Note: take a look and metrics API maybe it could be useful.