Document consul sd tls_config (#3440)
Document consul sd tls_config (#3440)

Fixes https://github.com/prometheus/docs/issues/681
Gouthamve/prometheus,Gouthamve/prometheus,AlekSi/prometheus,AlekSi/prometheus,GoogleCloudPlatform/prometheus,amorken/prometheus,amorken/prometheus,kangwoo/prometheus,amorken/prometheus,alileza/prometheus,brancz/prometheus,alileza/prometheus,Conorbro/prometheus,prometheus/prometheus,Conorbro/prometheus,AlekSi/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,Conorbro/prometheus,alileza/prometheus,keegancsmith/prometheus,brancz/prometheus,Gouthamve/prometheus,brancz/prometheus,GoogleCloudPlatform/prometheus,Stackdriver/stackdriver-prometheus,kangwoo/prometheus,keegancsmith/prometheus,Stackdriver/stackdriver-prometheus,kangwoo/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,brancz/prometheus,Stackdriver/stackdriver-prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,Conorbro/prometheus,prometheus/prometheus,AlekSi/prometheus,amorken/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,Stackdriver/stackdriver-prometheus