retrieval: fix memory leak in scrape cache
retrieval: fix memory leak in scrape cache

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