Metric Aggregator uses dedicated thread and hence should catch all exceptions

[cijothomas]

MetricAggregator is the only place in this SDK which creates a new Thread for running background task. The code should be protected from ever throwing unhandled exception as it could crash the application. While no existing occurences are reported, this should be done to avoid issues like microsoft/ApplicationInsights-dotnet-server#1088

this.aggregationThread = new Thread(this.Run);