Update empty message filtering to catch formatted string case

[o-nikolas]

Overview

• Empty message filtering added in 2.0 does not catch the edge case
  where the empty message is delivered as a formatted string (see repro example below).

• Update tests to catch future regressions of this case

• Update .gitignore to ignore the coverage file created by make tests

---

repro

from watchtower import CloudWatchLogHandler
import logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)
logger.addHandler(CloudWatchLogHandler(log_group='test_empty_msg_json',
                                                  use_queues=True))

logger.info("foo")
logger.info("%s", '')
logger.info("bar")

Running the above script yields this failure, where the empty message fails cloudwatch local validation and the whole batch of logs are not emitted to cloudwatch:

INFO:botocore.credentials:Found credentials in shared credentials file: ~/.aws/credentials
INFO:__main__:foo
INFO:__main__:
INFO:__main__:bar
/home/ANT.AMAZON.COM/onikolas/code/oss/watchtower/o-nikolas_watchtower/watchtower/watchtower.py:330: WatchtowerWarning: Failed to deliver logs: Parameter validation failed:
Invalid length for parameter logEvents[1].message, value: 0, valid min length: 1
  warnings.warn("Failed to deliver logs: {}".format(e), WatchtowerWarning)
/home/ANT.AMAZON.COM/onikolas/code/oss/watchtower/o-nikolas_watchtower/watchtower/watchtower.py:334: WatchtowerWarning: Failed to deliver logs: None
  warnings.warn("Failed to deliver logs: {}".format(response), WatchtowerWarning)

Running with the changes in this PR yields the expected behaviour of just emitting the warning for empty messages, but the other two valid messages are still sent to cloudwatch:

INFO:botocore.credentials:Found credentials in shared credentials file: ~/.aws/credentials
INFO:__main__:foo
/home/ANT.AMAZON.COM/onikolas/code/oss/watchtower/o-nikolas_watchtower/watchtower/watchtower.py:349: WatchtowerWarning: Received empty message. Empty messages cannot be sent to CloudWatch Logs
  warnings.warn("Received empty message. Empty messages cannot be sent to CloudWatch Logs", WatchtowerWarning)
INFO:__main__:
INFO:__main__:bar

[kislyuk]

Thanks for your contribution @o-nikolas. Will cut a release shortly.

[kislyuk]

Released in v2.0.1.

[o-nikolas]

Thanks Andrey, I appreciate your ongoing support for this project!