Add benchmark for mergedSeriesSet
Add benchmark for mergedSeriesSet

Gouthamve/tsdb,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,kangwoo/prometheus,alileza/prometheus,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,prometheus/tsdb,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,keegancsmith/prometheus