Add unittest for federation external_labels behaviour
Add unittest for federation external_labels behaviour

JavierZunzunegui/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,brancz/prometheus,prometheus/prometheus,kangwoo/prometheus,Stackdriver/stackdriver-prometheus,yanghongkjxy/prometheus,prometheus/prometheus,yanghongkjxy/prometheus,AlekSi/prometheus,keegancsmith/prometheus,amorken/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,amorken/prometheus,keegancsmith/prometheus,brancz/prometheus,prometheus/prometheus,yanghongkjxy/prometheus,Stackdriver/stackdriver-prometheus,Gouthamve/prometheus,AlekSi/prometheus,prometheus/prometheus,kangwoo/prometheus,amorken/prometheus,AlekSi/prometheus,tomwilkie/prometheus,dsymonds/prometheus,dsymonds/prometheus,Colstuwjx/prometheus,JavierZunzunegui/prometheus,Stackdriver/stackdriver-prometheus,Conorbro/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,SaltedMan/prometheus-annotated,AlekSi/prometheus,yanghongkjxy/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,amorken/prometheus,dsymonds/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,SaltedMan/prometheus-annotated,alileza/prometheus,tomwilkie/prometheus,Conorbro/prometheus,SaltedMan/prometheus-annotated,GoogleCloudPlatform/prometheus,Stackdriver/stackdriver-prometheus,brancz/prometheus,tomwilkie/prometheus,JavierZunzunegui/prometheus,Conorbro/prometheus,brancz/prometheus,JavierZunzunegui/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,SaltedMan/prometheus-annotated,Conorbro/prometheus,kangwoo/prometheus,Colstuwjx/prometheus,tomwilkie/prometheus,dsymonds/prometheus,alileza/prometheus,Colstuwjx/prometheus,Colstuwjx/prometheus