Emit metrics for notification delivery time

[pjenvey]

Notifications include a created at time (the timestamp field). Assuming the endpoint and connection nodes clocks are accurate/in sync, we should be able to track the total time taken from notification send to its delivery.

Should this be emitted when the notification is sent or when it's finally ACK'd?

(Remote Settings has begun tracking its delivery times on its own side as part of its pipeline checks, so this would be nice to have on our side).