Fix #2965
Fix #2965

We would overscan when hitting a value directly, interspersed with
samples in between timestamps. Apparently, that happens rarely enough
that it was only noticed recently.

yanghongkjxy/prometheus,brancz/prometheus,amorken/prometheus,prometheus/prometheus,Gouthamve/prometheus,kangwoo/prometheus,amorken/prometheus,prometheus/prometheus,alileza/prometheus,prometheus/prometheus,Stackdriver/stackdriver-prometheus,Colstuwjx/prometheus,brancz/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,Conorbro/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,yanghongkjxy/prometheus,keegancsmith/prometheus,AlekSi/prometheus,alileza/prometheus,JavierZunzunegui/prometheus,brancz/prometheus,keegancsmith/prometheus,alileza/prometheus,JavierZunzunegui/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus,kangwoo/prometheus,yanghongkjxy/prometheus,Colstuwjx/prometheus,Colstuwjx/prometheus,Gouthamve/prometheus,amorken/prometheus,Stackdriver/stackdriver-prometheus,Gouthamve/prometheus,AlekSi/prometheus,Stackdriver/stackdriver-prometheus,alileza/prometheus,AlekSi/prometheus,Stackdriver/stackdriver-prometheus,Conorbro/prometheus,brancz/prometheus,Conorbro/prometheus,kangwoo/prometheus,JavierZunzunegui/prometheus,AlekSi/prometheus,amorken/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,JavierZunzunegui/prometheus,yanghongkjxy/prometheus,Conorbro/prometheus,Colstuwjx/prometheus