this commit performs a small refactor to one of the unit tests in
`linkerd-stack`'s load-shedding middleware.

this adds a span to the worker tasks spawned in this test, so that
tracing logs can be associated with particular oneshot services.

see #3744 for more information on upgrading our tower dependency. this
is cherry-picked from investigations on that branch related to breaking
changes in 0.5 related to the `Buffer` middleware.

after this change, logs now look like this:

```
; RUST_LOG="trace" cargo test -p linkerd-stack buffer_load_shed -- --nocapture

running 1 test
[     0.002770s] TRACE worker{id=oneshot1}: tower::buffer::service: sending request to buffer worker
[     0.002809s] TRACE worker{id=oneshot2}: tower::buffer::service: sending request to buffer worker
[     0.002823s] TRACE worker{id=oneshot3}: tower::buffer::service: sending request to buffer worker
[     0.002843s] DEBUG worker{id=oneshot4}: linkerd_stack::loadshed: Service has become unavailable
[     0.002851s] DEBUG worker{id=oneshot4}: linkerd_stack::loadshed: Service shedding load
[     0.002878s] TRACE tower::buffer::worker: worker polling for next message
[     0.002885s] TRACE tower::buffer::worker: processing new request
[     0.002892s] TRACE worker{id=oneshot1}: tower::buffer::worker: resumed=false worker received request; waiting for service readiness
[     0.002901s] DEBUG worker{id=oneshot1}: tower::buffer::worker: service.ready=true processing request
[     0.002914s] TRACE worker{id=oneshot1}: tower::buffer::worker: returning response future
[     0.002926s] TRACE tower::buffer::worker: worker polling for next message
[     0.002931s] TRACE tower::buffer::worker: processing new request
[     0.002935s] TRACE worker{id=oneshot2}: tower::buffer::worker: resumed=false worker received request; waiting for service readiness
[     0.002946s] TRACE worker{id=oneshot2}: tower::buffer::worker: service.ready=false delay
[     0.002983s] TRACE worker{id=oneshot5}: tower::buffer::service: sending request to buffer worker
[     0.003001s] DEBUG worker{id=oneshot6}: linkerd_stack::loadshed: Service has become unavailable
[     0.003007s] DEBUG worker{id=oneshot6}: linkerd_stack::loadshed: Service shedding load
[     0.003017s] DEBUG worker{id=oneshot7}: linkerd_stack::loadshed: Service has become unavailable
[     0.003024s] DEBUG worker{id=oneshot7}: linkerd_stack::loadshed: Service shedding load
[     0.003035s] TRACE tower::buffer::worker: worker polling for next message
[     0.003041s] TRACE tower::buffer::worker: resuming buffered request
[     0.003045s] TRACE worker{id=oneshot2}: tower::buffer::worker: resumed=true worker received request; waiting for service readiness
[     0.003052s] DEBUG worker{id=oneshot2}: tower::buffer::worker: service.ready=true processing request
[     0.003060s] TRACE worker{id=oneshot2}: tower::buffer::worker: returning response future
[     0.003068s] TRACE tower::buffer::worker: worker polling for next message
[     0.003073s] TRACE tower::buffer::worker: processing new request
[     0.003077s] TRACE worker{id=oneshot3}: tower::buffer::worker: resumed=false worker received request; waiting for service readiness
[     0.003084s] DEBUG worker{id=oneshot3}: tower::buffer::worker: service.ready=true processing request
[     0.003091s] TRACE worker{id=oneshot3}: tower::buffer::worker: returning response future
[     0.003099s] TRACE tower::buffer::worker: worker polling for next message
[     0.003103s] TRACE tower::buffer::worker: processing new request
[     0.003107s] TRACE worker{id=oneshot5}: tower::buffer::worker: resumed=false worker received request; waiting for service readiness
[     0.003114s] DEBUG worker{id=oneshot5}: tower::buffer::worker: service.ready=true processing request
[     0.003121s] TRACE worker{id=oneshot5}: tower::buffer::worker: returning response future
[     0.003129s] TRACE tower::buffer::worker: worker polling for next message
test loadshed::tests::buffer_load_shed ... ok
```

Signed-off-by: katelyn martin <[email protected]>