Add missing import to promtool, fix build (#4395)
Add missing import to promtool, fix build (#4395)

Sorry, I used GitHub's web-based merge-conflict-resolution editor on
https://github.com/prometheus/prometheus/pull/4308 and it didn't show me
test errors afterwards, but maybe they didn't run again or I should have
waited or something.

Signed-off-by: Julius Volz <d7748b1e6f41fd74500c5ccae0980eab88c2ef61@gmail.com>
prometheus/prometheus,alileza/prometheus,Gouthamve/prometheus,alileza/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,kangwoo/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus