sentinel-dashboard 的监控数据持久化扩展方式希望可以更优雅一些

Issue Description

Type: feature request

Describe what happened (or what feature you want)

sentinel-dashboard的监控数据持久化扩展方式必须通过修改源码的替换 MetricsRepository 的实现

// Sentinel-dashboard's monitoring data persistence extension must be implemented by modifying the source code to replace MetricsRepository

Describe what you expected to happen

可以实现类似 Spring Boot 条件bean，只在不存在用户自定义的 MetricsRepository 实现时采用 InMemoryMetricsRepository 实现。

好处有：

1. 自定义实现时用户可以在 sentinel-dashboard 中添加一个自定义实现的jar（META-INF/spring.factories 中实现自动注入其他持久化实现）即可实现数据持久化的切换。
2. 这样 sentinel-dashboard 升级后，只需要引入jar，重新打包即可，无需修改sentinel-dashboard源码，更进一步，用户可以不用重新打包，在启动命令中将该自定义jar加入类路径这样可以实现sentinel-dashboard与自定义jar分离，升级无任何依赖。

// English translation:

Mechanism like Spring Boot conditional bean can be implemented, using the InMemoryMetricsRepository implementation only when there is no user-defined MetricsRepository implementation.

The benefits are:

1. In the custom implementation, users can add a custom implementation jar in the sentinel-dashboard (automatically inject other persistence implementations in META-INF/spring.factories) to achieve data persistence switching.
2. After the upgrade of the sentinel-dashboard, you only need to introduce the jar and repackage it. You don't need to modify the sentinel-dashboard source code. Further, the user can repackage the custom jar in the startup command. Sentinel-dashboard is separated from the custom jar, the upgrade has no dependencies.