First Deadline Reported Metric

[ghost]

Is your feature request related to a problem? Please describe.
Currently there is no way to see how long a round has been ongoing before the first deadline was received by the pool. It would be helpful to see how long it took for first deadline to be received on a certain upstream. The more chains being mined make this information more relevant to see.

Describe the solution you'd like
Add a new row in each proxy upstream that displays "First dl received after xx s" or even better: "First dl: xx s (xx s average round)" with average round being tied to historicalRoundsToKeep.

Only problem I see with using historicalRoundsToKeep is if the round passes without a deadline what value would you use for an average?