ApplicationInsightsTraceListener does not respect Flush

[markanewman]

When calling Trace.Flush(), some of the last messages may be lost. Per this, TelemetryClient.Flush needs to be called to make sure the last batch is sent.

[kavanj]

A similar issue exists in ApplicationInsightsAppender (Log4NetAppender).
ApplicationInsightsAppender doesn't override AppenderSkeleton.Flush(), so calling Flush just hits AppenderSkeleton.Flush().

I haven't experimented with ApplicationInsightsTarget (NLogTarget), but I'd expect the same issue to exist there, since there doesn't appear to be anything calling TelemetryClient.Flush()

[markanewman]

I am not familiar with NLog, but I looked for AppenderSkeleton.Flush() before I put in my PR and was unable to find it. The version that is referenced doesn't implement the IFlushable interface in the latest code. To my eye, this seems to be the issue with #68.