Stimulus id contextvars explicit handlers

[sjperkins]

• Closes Transition tracing for scheduler task transitions #5849
• Tests added / passed
• Passes pre-commit run --all-files

[sjperkins]

@fjetter Would you mind taking a brief look at the approach here and giving me your opinion on the overall complexity required here? I think looking at scheduler.py and test_scheduler.py will give you 95% of the flavour.

Essentially, explicit handlers requiring stimulus_id's are created that call implementations.

So for e.g. there's an explicit handle_update_graph function that calls update_graph. In practice this means that more stimulus_id's must be supplied in the test cases (and client.py). Also many of the tests are modified to call their handler instead of the implementation e.g. s.update_graph(...) changes to s.handle_update_graph(..., stimulus_id="test")

[sjperkins]

This is based on #6046

[fjetter]

I believe we should not involve the Client at this point. There is almost no additional information avialable if the client generates an ID or if the scheduler just generates it.
We wouldn't need to define this many handlers if the client is not involved

[sjperkins]

We are sending other stimuli from the client though? e.g. key release stimuli?

With handlers explicitly requiring stimuli to be sent in this PR, it's necessary to do so in other places in the Client too. Why not complete the loop on all stimuli?

[sjperkins]

This can be removed

[sjperkins]

Pehaps check_worker_ttl should be decorated with expects_stimulus? I guess an initialising stimulus could be provided when setting up the PeriodicCallback

[github-actions]

Unit Test Results

       18 files  ±  0         18 suites  ±0   9h 19m 39s ⏱️ - 1m 55s
  2 726 tests +  2    2 640 ✔️  -   2       82 💤 +1  4 ❌ +3 
24 382 runs  +18  23 160 ✔️ +11  1 218 💤 +4  4 ❌ +3 

For more details on these failures, see this check.

Results for commit bfcbdb0. ± Comparison against base commit 6e30766.

♻️ This comment has been updated with latest results.

[sjperkins]

Difficult to get ContextVar interaction with Tornado right in add_callback methods. See for e.g. tornadoweb/tornado#2731. Closing in favour of #6095