Make DB blocks list public.
Make DB blocks list public.

This enable computing the first timestamp in the DB
only accessing public interfaces.

Signed-off-by: Thibault Chataigner <760b602e41777d70d0be9acf5dc7a32f8153d50b@criteo.com>

kangwoo/prometheus,alileza/prometheus,kangwoo/prometheus,prometheus/prometheus,Gouthamve/prometheus,kangwoo/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/tsdb,alileza/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,Gouthamve/tsdb,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus