Add prometheus metrics exposure variable

[im-jinxinwang]

nacos version： all
添加prometheus metrics 变量启动的原因：
在快速构建 Nacos 环境时，需要将 application.properties 文件从 image 中复制出来，并添加监控指标暴露，然后按照 configmap 将其挂载进去。这种做法增加了快速上手的复杂性。

之前还使用过下面的这种方法：

        - name: NACOS_AUTH_ENABLE
          value: "true  -Dmanagement.endpoints.web.exposure.include=*"

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