Abbreviate CHANGELOG.md entry for 1.2.0
Abbreviate CHANGELOG.md entry for 1.2.0

It now follows the policy of only documenting user-relevant changes.

GoogleCloudPlatform/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Stackdriver/stackdriver-prometheus,alileza/prometheus,tattsun/prometheus,Conorbro/prometheus,Colstuwjx/prometheus,PrFalken/prometheus,Gouthamve/prometheus,Colstuwjx/prometheus,amorken/prometheus,yanghongkjxy/prometheus,AlekSi/prometheus,Conorbro/prometheus,JavierZunzunegui/prometheus,agaoglu/prometheus,alileza/prometheus,slok/prometheus,tomwilkie/prometheus,kangwoo/prometheus,AlekSi/prometheus,gottwald/prometheus,alileza/prometheus,kangwoo/prometheus,kzittritsch/prometheus,keegancsmith/prometheus,alileza/prometheus,JavierZunzunegui/prometheus,datajet-io/prometheus,GoogleCloudPlatform/prometheus,tattsun/prometheus,AlekSi/prometheus,gottwald/prometheus,brancz/prometheus,SaltedMan/prometheus-annotated,tomwilkie/prometheus,yanghongkjxy/prometheus,adamron1/prometheus,Colstuwjx/prometheus,tattsun/prometheus,agaoglu/prometheus,Gouthamve/prometheus,slok/prometheus,brancz/prometheus,PrFalken/prometheus,Conorbro/prometheus,prometheus/prometheus,adamron1/prometheus,kzittritsch/prometheus,dsymonds/prometheus,JavierZunzunegui/prometheus,datajet-io/prometheus,GoogleCloudPlatform/prometheus,adamron1/prometheus,yanghongkjxy/prometheus,SaltedMan/prometheus-annotated,Gouthamve/prometheus,adamron1/prometheus,JavierZunzunegui/prometheus,kzittritsch/prometheus,dsymonds/prometheus,kangwoo/prometheus,PrFalken/prometheus,prometheus/prometheus,agaoglu/prometheus,keegancsmith/prometheus,tomwilkie/prometheus,slok/prometheus,prometheus/prometheus,PrFalken/prometheus,yanghongkjxy/prometheus,amorken/prometheus,amorken/prometheus,keegancsmith/prometheus,Stackdriver/stackdriver-prometheus,prometheus/prometheus,dsymonds/prometheus,datajet-io/prometheus,slok/prometheus,Stackdriver/stackdriver-prometheus,Conorbro/prometheus,tattsun/prometheus,Stackdriver/stackdriver-prometheus,keegancsmith/prometheus,Gouthamve/prometheus,SaltedMan/prometheus-annotated,kangwoo/prometheus,amorken/prometheus,AlekSi/prometheus,gottwald/prometheus,alileza/prometheus,dsymonds/prometheus,GoogleCloudPlatform/prometheus,brancz/prometheus,datajet-io/prometheus,Colstuwjx/prometheus,kzittritsch/prometheus,gottwald/prometheus,tomwilkie/prometheus,SaltedMan/prometheus-annotated,brancz/prometheus,keegancsmith/prometheus,agaoglu/prometheus