Optimise labels.Compare (#6539)
Optimise labels.Compare (#6539)

strings.Compare isn't meant to be used, and this way we save one
comparison which is thus very slightly cheaper.

benchmark                                     old ns/op     new ns/op delta
BenchmarkPostingsForMatchers/Head/n="1"-4     236305440     233515705 -1.18%

Signed-off-by: Brian Brazil <aadf89994a343dc176a47081f0c0bbea26f0a345@robustperception.io>
Gouthamve/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus