discovery/dns: add test case for SDConfig.UnmarshalYAML. (#6035)
discovery/dns: add test case for SDConfig.UnmarshalYAML. (#6035)

* discovery/dns: Add code coverage.

Signed-off-by: johncming <64aa7c399bffb85fffaccf95be9acb9553c38de5@yahoo.com>

* discovery/dns: add test case for SDConfig.UnmarshalYAML.

Signed-off-by: johncming <64aa7c399bffb85fffaccf95be9acb9553c38de5@yahoo.com>

kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus