feat: support metric of sent_to_receiver or events_matched

[seethedoor]

Hi,
Now we have metric of event_exporter_events_sent to calculate the total number of all events that event-exporter received from list&watch of api-server. But now if we want to know how many events that matched all rules and sent to receivers, there is no metrics to show this.
I think we need a metric named like event_exporter_events_sent_to_receiver or event_exporter_events_matched to support this kind of information.
thx.