DEV: Expose AI spam scanning metrics

[nattsw]

This should give us a better idea on how our scanner is faring across sites.

# HELP discourse_discourse_ai_spam_detection AI spam scanning statistics
# TYPE discourse_discourse_ai_spam_detection counter
discourse_discourse_ai_spam_detection{db="default",type="scanned"} 16
discourse_discourse_ai_spam_detection{db="default",type="is_spam"} 7
discourse_discourse_ai_spam_detection{db="default",type="false_positive"} 1
discourse_discourse_ai_spam_detection{db="default",type="false_negative"} 2

Note: I considered using DiscoursePluginRegistry.register_global_collector but due to the implementation, it seems to only work well with Gauge-type collectors that emit the value directly in collect.

[romanrizzi]

Looks good to me! Suggested a change to fix CI