Request to add prometheus monitoring function

[ilanni2460]

Request to add prometheus monitoring function，You can refer to this link：
https://github.com/mohsenmottaghi/docker-openresty-prometheus

[neomantra]

It looks like that person's tooling uses this library:
https://github.com/knyar/nginx-lua-prometheus

Beyond the build scaffolding in that repo, there are no extra modules or compile-time changes needed.

There is an OPM package for it, so you can install it in your own derived image:

• OPM Package: https://opm.openresty.org/package/knyar/nginx-lua-prometheus/
• Docs: https://github.com/openresty/docker-openresty#opm
• Example: https://github.com/openresty/docker-openresty/blob/master/buster/Dockerfile.opm_example