Guard against tags being nil in EC2 discovery
Guard against tags being nil in EC2 discovery

Fixes #3001

alileza/prometheus,amorken/prometheus,Gouthamve/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,AlekSi/prometheus,Conorbro/prometheus,Stackdriver/stackdriver-prometheus,brancz/prometheus,kangwoo/prometheus,AlekSi/prometheus,Conorbro/prometheus,keegancsmith/prometheus,Stackdriver/stackdriver-prometheus,brancz/prometheus,Gouthamve/prometheus,kangwoo/prometheus,kangwoo/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,amorken/prometheus,Gouthamve/prometheus,brancz/prometheus,keegancsmith/prometheus,AlekSi/prometheus,prometheus/prometheus,kangwoo/prometheus,amorken/prometheus,prometheus/prometheus,amorken/prometheus,brancz/prometheus,AlekSi/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Conorbro/prometheus,Conorbro/prometheus,Stackdriver/stackdriver-prometheus,keegancsmith/prometheus,prometheus/prometheus,Stackdriver/stackdriver-prometheus