Add in additional tests for metrics and/or labels called start/end.
Add in additional tests for metrics and/or labels called start/end.

Signed-off-by: Clayton Peters <e0d536682af7e52764ef29450955f520960a061f@man.com>

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