Store current time when TaskResult started

[yktakaha4]

Thank you to all maintainers of this great OSS.

Referring to the issue below, I generate TaskResult when Task is PENDING.
ref: #286 (comment)

This code works fine, but I can't use date_created as the start time of the Task because the TaskResult already exists when STARTED is recorded.
Additionally, in my use case, I want to measure the time from when a task is added to the message broker until the worker starts processing it.

By recording date_started as the time associated with STARTED, we can calculate the waiting time in the queue or the actual processing time of the worker.
Use django.db.models.functions.Now to get the current time to avoid being affected by the server's time settings.
ref: https://docs.djangoproject.com/en/5.0/ref/models/database-functions/#now

This PR is backward compatible because it simply adds a column and sets a value.
For users who do not create TaskResults themselves, date_created and date_started have the same meaning.

Testing

We can check the value of date_started in django admin.

For testing, I implemented a 5 second sleep on @shared_task.
The time increases in the order date_created < date_started < date_done.

select id, status, date_created, date_started, date_done
from django_celery_results_taskresult
where id = 35;

id	status	date_created	date_started	date_done
35	SUCCESS	2024-06-08 06:14:37.631030 +00:00	2024-06-08 06:14:37.650764 +00:00	2024-06-08 06:14:43.334453 +00:00

Please let me know if there is anything else that should be tested.

[auvipy]

I like the intent of the PR. however I need more through review before merge it

[auvipy]

in the mean time, can you please review this PR? #407

[yktakaha4]

@auvipy Thank you for your quick review.

I have confirmed the fix for #407.
I noticed that no tests were written and no documentation was provided, but I couldn't think of any points that would functionally conflict with my PR.
(Should I point this out in the review?)

In my personal opinion, TaskResult.state is just for storing celery.states, and I thought it might make state customization possible in Celery and then change it, but I'm not confident.

[auvipy]

can we think of any example, using the new feature here?

[yktakaha4]

@auvipy Added example. However, I'm not good at English, so I would like you to correct the sentence.

By the way, you previously asked whether this package should include the behavior of registering TaskResult at states.PENDING timing.
ref: #286 (comment)

If it is possible for me, I would like to create another PR. What do you think?

[auvipy]

you can certainly open another PR. but what are the issues with the open PR?

[yktakaha4]

Sorry I'm not good at explaining. There is nothing problems with this PR.
I would like to submit a new PR for the PENDING status registration.
If there is no problem with this PR, could you please merge it?

[auvipy]

you are most welcome to come with a new PR