Add firewall stats

[klincool]

Hi team!

Thanks for useful exporter.

It would be great to add firewall statistics to the metrics.
Minimally I would like to see counters of negative events, such as block and all types of challenges. And ideally all types of events are of course.
Unfortunately, I can't make a pull request.

Thanks.

[martinhaus]

Hey, we currently have cloudflare_zone_firewall_events_count and cloudflare_zone_health_check_events_origin_count metrics.
I don't have a good way to test other metrics right now, any contribution is welcome.