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

[incertum]

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

#3194 (comment)

[incertum]

CC @sgaist and @FedeDP

/milestone 0.42.0

[poiana]

Issues go stale after 90d of inactivity.

Mark the issue as fresh with /remove-lifecycle stale.

Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Provide feedback via https://github.com/falcosecurity/community.

/lifecycle stale

[leogr]

/milestone 0.43.0

[poiana]

Stale issues rot after 30d of inactivity.

Mark the issue as fresh with /remove-lifecycle rotten.

Rotten issues close after an additional 30d of inactivity.

If this issue is safe to close now please do so with /close.

Provide feedback via https://github.com/falcosecurity/community.

/lifecycle rotten

[leogr]

/remove-lifecycle rotten

[poiana]

Issues go stale after 90d of inactivity.

Mark the issue as fresh with /remove-lifecycle stale.

Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Provide feedback via https://github.com/falcosecurity/community.

/lifecycle stale