fix: add process logs & flush() returns future

[Mercy811]

Summary

• send events without submitting to threads pool
• add logs: amplitude.log

Checklist

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[Mercy811]

Since we already have a callback function where users can define logs they want to see. If I add a default logger to output every piece of logs in the terminal, it's gonna be a lot. So I store logs to a file amplitude.log.

For flush() function in worker.py, send events directly instead of submitting them to threads pool. If events are sent to another thread, the following situation may happen: in one thread, following code is executed by order; in the other thread, events are being processed.

In this case, I got errors in test_worker.py everywhere workers.flush() exits, but all unit tests are passed in github where python -m unittest discover -s ./src -p 'test_*.py' is executed. The reason behind this is probably because code is executed faster in laptop than in github. And when to test re-try logic, we verify the number of events, so to get the right/expected number of events, we have to wait the previous event process done.

[kevinpagtakhan]

@bohan-amplitude are we ok with this?

[bohan-amplitude]

The difference is flush event using main thread or threadpool. I was changing this because I don't want to block main thread. Right now many unittests need to wait flush finish then verify result. I talked with Xinyi and change back to send won't cause problem for most of the time. I think here it's better to let flush() return future object returned by threads_pool.submit(). In test, use the future object to wait for result before verify. In other use cases returned future can be ignored. @Mercy811 Can you take a look at this and try?

[bohan-amplitude]

Since timeline could have multiple destination plugins, return here may end the function earlier and only return future of first flush call. Should return a list of future for all destination plugins

[bohan-amplitude]

unused import

[bohan-amplitude]

Extra indent here. Otherwise look good to me.