Adding gauge for number of open head chunks.
Adding gauge for number of open head chunks.

Fixes #1710

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