Add email receiver metrics

What does this MR do?

Create a metric event when we handle an incoming email. The event is tagged with the handler, and, if applicable, the project.

Are there points in the code the reviewer needs to double check?

I think this is pretty straightforward.

Why was this MR needed?

This would already be useful, but it's especially important with the release of Service Desk, as anyone can enable that on their project. We can build a similar table to https://performance.gitlab.net/dashboard/db/events?panelId=32&fullscreen&orgId=1.