Collect oTel sub-spans for interesting events

[cohix]

Right now we collect a sngle trace for the entire request in proxy mode.

We should instrument further with sub-spans to collect data like:

• How long from the start of request processing did it take to start executing the sequence?
• How long did the call to grav.Tunnel take?
• How long did it take to execute each step in a sequence?
• How long from the end of the sequence did it take to respond to the request?

It may not be possible to add all of these without a refactor, but they would be useful things to have.