[Prometheus metrics gaps] `num_evts` metric still missing in the Prometheus output

Motivation

The metric num_evts which is available for the other metric output sinks is still missing in the Prometheus output.

Feature

Consider adding this missing metric.

Alternatives

Decide to not support it for Prometheus.

Additional context

https://github.com/falcosecurity/falco/issues/3194#issuecomment-2111009270