Output full stack trace when async sender dies

[tillig]

I'm using this gem in conjunction with the fluentd-plugin-application-insights gem. However, when trying to use this in a fluentd DaemonSet on Kubernetes, I get this:

E, [2019-07-19T21:18:07.752463 #20580] ERROR -- application_insights: Asynchronous sender work thread terminated abnormally: "\x92" from ASCII-8BIT to UTF-8

The problem is... that doesn't tell me where in the 50-ish line send block this is happening. I can't fix it because I can't figure out the actual source of the error.

I found the line where this is getting thrown but it would be more helpful to provide an actual stack trace of the source of the issue (maybe in debug-level logging if you don't want it in the default error message) so things can be traced down.

Since I can only see this replicated in a mixed Linux/Windows AKS cluster, right now the only way I can troubleshoot it is to fork both this gem and the fluentd-plugin-application-insights gem, add the logging I need, then update a custom Docker image to use those. (Sorta painful. Not impossible, but definitely some work that could be saved with some better logging.)