Cleanup span usage to track wait time. (#7581)
Cleanup span usage to track wait time. (#7581)

- Span methods are nil safe, so we can use the same pattern as in the activator to have a "nil" span and call the usual methods on it without having to create dummy functions.
- We don't want the context to propagate into the request as by then, the wait is already over. It's not a nested span at this point.
knative/serving,knative/serving