unregister source when the target is empty
unregister source when the target is empty

Signed-off-by: Augustin Husson <180f9fec3975da6d787681c97a0338d053288e4a@gmail.com>

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