feat: add prometheus

[evan361425]

Description by https://github.com/knyar/nginx-lua-prometheus

Usually it will be used by:

local prometheus = require("prometheus").init("prometheus_metrics")

not sure how to type the required module, helps needed.

[sumneko]

not sure how to type the required module, helps needed.

Add return PrometheusLib at the bottom of the file.

[evan361425]

@sumneko fixed done, thanks!