Add some unit tests for funcs in pkg/labels/labels.go (#7116)
Add some unit tests for funcs in pkg/labels/labels.go (#7116)

This PR is about adding some unit tests for funcs in pkg/labels/labels.go.

Signed-off-by: Hu Shuai <a4797ccdc3c71d8ffc5c17a5bb7f114e3818f278@cn.fujitsu.com>
alileza/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,kangwoo/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,prometheus/prometheus,prometheus/prometheus