[SQL] Renamed ColumnStat to ColumnMetrics to avoid confusion between ColumnStats

Class names of these two are just too similar.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2189 from liancheng/column-metrics and squashes the following commits:

8bb3b21 [Cheng Lian] Renamed ColumnStat to ColumnMetrics to avoid confusion between ColumnStats