discovery/consul: fix leaking goroutine from test
discovery/consul: fix leaking goroutine from test

Signed-off-by: Mateusz Gozdek <13f764135ff35d07236cdb9e39f2edb4395b2583@gmail.com>

alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus