auto balance hot regions across regionservers

[pidren]

It would be nice since the operator has (1) access to hbase metrics, especially per region get/scan stats, (2) already has the ability to selectively move regions between regionservers for the operator to check per controller iteration to check for regionservers with hot regions and spread the regions across other regionservers with only best effort guarantee.

[dethi]

HBase loadbalancer already does that.