Watchtower

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- Format message placeholders when using add_log_record_attrs (184)

- Remove “:” in program_name of stream name (207)

- Add note about strftime format string being required

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- Fix type errors introduced in 201 (required to restore compatibility
with Python 3.8)

- Test infrastructure improvements

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- Support log group tagging (201)

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- Use timeout when waiting for queues to empty at shutdown

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- Avoid crashing flush() when CreateLogStream API call is throttled
(192)

- Replace deprecated function datatime.utcnow() (196)

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- Truncate messages based on bytes, not unicode characters (181)

- Test suite improvements (180, 182)