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.

@godfat @felipe_artur @stanhu - if we merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1662, then we should also include this.

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• 4e7ff7c2 - Store projects in metrics for email replies

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• 19970e1d - Store projects in metrics for email replies

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approved this merge request

merged

mentioned in commit cc9e92a0

Picked into 9-1-stable, will go into 9.1.0-rc6

mentioned in commit 08edb752

resolved all discussions