Add support for callback metrics

[knyar]

When gauge metric values can be set at collection time, we don't really need to store them in the shared dict or even in per-worker local state. For cases like this, it might be helpful to support metrics that call a function at collection time, similar to callback metrics in Java or custom collectors in Python.

More context: https://github.com/knyar/nginx-lua-prometheus/issues/168#issuecomment-2048965469