Fix golint warning in labels.go (#6839)
Fix golint warning in labels.go (#6839)

Signed-off-by: Guangwen Feng <98e12d19818083ae8a98303786e6a151211377ea@cn.fujitsu.com>
Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,kangwoo/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,Gouthamve/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus