Add metrics API to NexusGate

[pescn]

我们需要对NexusGate运行状态中的数据，返回一个用于遥测的接口，定义为 /metrics 接口，返回格式为 Prometheus 使用的标准格式

字段方面，请参照Helicone的Dashboard界面上面的指标进行统计

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We need to return an interface for telemetering the data in the running state of NexusGate, defined as the /metrics interface, with the return format being the standard format used by Prometheus.

For the fields, please refer to the metrics on the Helicone Dashboard interface for statistics