Log messages should use arguments rather than fully-formatted strings

[lbernick]

Hello! My organization is using backoff version 1.8.0, and would benefit from having log messages pass variable data in as arguments (e.g. logger.error("Foo: %s", "bar")) rather than logging fully formatted strings (e.g. logger.error("Foo: {}".format("bar"))). This style of logging is recommended by the python docs because it allows log sinks to process log messages in a more meaningful way. Specifically, my organization uses Sentry, which will understand that multiple calls with the same log message but different arguments represent different incidents of the same issue, but can't recognize logs as related to the same issue if arguments are pre-formatted before the string is logged. I am happy to make a pull request for this if you would like.

[bgreen-litl]

Sorry for the slow response. If you make a pull request for this I'll take a look at it. As long as this doesn't break existing code or logging behavior (I think it shouldn't) then it should be okay.

Also any changes need to work in Python 2.7, 3.5, 3.6, 3.7 (and probably 3.8).

Thanks!

[bgreen-litl]

PR #82 is merged and is included in backoff==1.8.1! Thanks for this.