pdpb: support to report bucket (#866)

* add ReportRegionStats

Signed-off-by: 5kbpers <[email protected]>

* rename rpc

Signed-off-by: 5kbpers <[email protected]>

* update go generating

Signed-off-by: 5kbpers <[email protected]>

* resolve conflicts

Signed-off-by: 5kbpers <[email protected]>

* report buckets

Signed-off-by: 5kbpers <[email protected]>

* add comment

Signed-off-by: 5kbpers <[email protected]>

* address comment

Signed-off-by: 5kbpers <[email protected]>

Author: 5kbpers