Include exception stack trace in 'message'

[johnpaulett]

Problem: Since switching from the builtin log handlers to fluent-logger-python, when an exception occurs in my web application, I only receive the single line from LogRecord.msg (e.g. {"message": "Internal Server Error: /"). The traceback and root exception are not present, making debugging nearly impossible.

Solution: In this PR, when we do not have a dict or JSON message, instead of using LogRecord.msg we call logging.Formatter.format(). For existing debug/info/warn/error string messages, this will result in no change, but when there is an exception, it will include the exc_text in the message, as is common with the builtin logging Formatters. For example: {'message': 'it failed\nTraceback (most recent call last):\n File "[..]/fluent-logger-python/tests/test_handler.py", line 363, in test_exception_message\n raise Exception(\'sample exception\')\nException: sample exception'}

Alternative Solutions:

• As mentioned in the README, the formatter could be configured with 'stack_trace': '%(exc_text)s'. The downside to this approach is that 1) the majority of log statements which lack an exception will have an unnecessary "stack_trace": "None", 2) it is surprising behavior to new library users that the message does not include the normal trace info.
• There have been prior PRs (Add formatted exception #19 and Add exception traceback handling to Formatter #38) which have manually used the traceback module to simulate what LogRecord.exc_text provides.

[coveralls]

Coverage remained the same at 100.0% when pulling 39e60c0 on johnpaulett:include-stack-trace into 268f577 on fluent:master.