TiUP无法编辑prometheus相关配置文件，如各告警规则配置文件

[topbrightwen]

TiDB升级到4.0后，改用TiUP来管理集群，但发现TiUP无法编辑prometheus相关配置文件，如各种告警规则配置文件，给管理造成很大不便，希望能增加这项功能。

[lucklove]

I think we can add an env variable to avoid covering user-edited configs in the startup script:

if [ $DONT_COVER_CONFIG == "true" ]; do
    cp {{.DeployDir}}/bin/prometheus/*.rules.yml {{.DeployDir}}/conf/
done

The variable can be added into the systems config file as this style:

monitoring_servers:
  - host: x.x.x.x
    env:
        - DONT_COVER_CONFIG: true

@lonng What do you think about this?

[lucklove]

Fixed by #712

[lonng]

Fixed by #712

@lucklove Better to link the documentation.