some refactor to make PostingsForMatchers more readable (#5897)
some refactor to make PostingsForMatchers more readable (#5897)

Signed-off-by: YaoZengzeng <24ab6c0308baa96f7d187360119779e115ef34f9@zju.edu.cn>
alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus