Minor cleanup of metrics.Source

- Added override.
- Marked some variables as private.

Author: Reynold Xin <rxin@apache.org>

Closes #1943 from rxin/metricsSource and squashes the following commits:

fbfa943 [Reynold Xin] Minor cleanup of metrics.Source. - Added override. - Marked some variables as private.

(cherry picked from commit eaeb0f76fa0f103c7db0f3975cb8562715410973)
Signed-off-by: Reynold Xin <rxin@apache.org>