Measure "time before new onion service becomes available"

Background

In recent DoS attacks onion service connection times suffered generally, but one especially strong effect was on the delay before new onion services became available, which grew to multiple minutes in some cases. This delay impacts testing of onion services and applications like Briar, Cwtch, and Quiet where users create onion services and almost immediately invite users to connect to them.

(I'm not sure about this, but it may have also affected the time for an onion service to become available after the Tor client hosting it went offline and returned online.)

Problem

"Time before new onion service becomes available" is not currently measured and reported on Tor Metrics. As a result, Tor developers, developers building on Tor, and onion service operators currently lack visibility into this problem when it arises. This lack of information limits their ability to diagnose problems, and it delays fixes.

Solution

Tor Metrics should measure "time before new onion service becomes available."

Also, if "time before existing but newly reconnected onion service becomes available" is a variable that behaves independently of "time before new onion service becomes available" then Tor Metrics should measure that too.