Native prometheus metrics

[jptosso]

Prometheus metrics are everywhere. We should support this as a first-class feature so our connectors can register and add WAF metrics to their applications.

This would benefit Cora Caddy and Wasm directly.

To do this, we could simply export a package that automatically registers the metrics or we could add helpers to add them at an application level.

[fzipi]

Related: #1008

[jcchavezs]

We first need to design the metrics to be tracked. That task would probably require people operating coraza e.g. @funkluk or @soujanyanmbri to propose interesting metrics and we all discuss them.

[jptosso]

It's hard to monitor all rule triggers. there are too many rules... what about having a filter to pick rules?
We also need to monitor disruptive actions
I found this reference https://github.com/XCiber/imperva-exporter/blob/d8c0c4af2652ce2f925e2f3ed0612d05346cf224/pkg/imperva/imperva.go