Feature request: latency metrics for bundle submission through the relay

[thegostep]

Searchers want to minimize the amount of time between when an opportunity becomes available and when they've placed a bid with the miners. As such, it would be useful to provide metrics around execution through an API or a dashboard which allows a user to see the amount of time between when a bundle was received by the relay and when it was delivered to the miners.

This is related to #29