Flush Application Insights on shutdown (#325)

Based off v3 jobs' [shutdown code](https://github.com/NuGet/NuGet.Services.Metadata/blob/master/src/Ng/Program.cs#L82). Fixes https://github.com/NuGet/Engineering/issues/1116

Author: loic-sharma

src/NuGet.Jobs.Common/JobRunner.cs

@@ -8,6 +8,7 @@
 using System.Linq;
 using System.Net;
 using System.Threading.Tasks;
+using Microsoft.ApplicationInsights.Extensibility;
 using Microsoft.Extensions.Logging;
 using NuGet.Services.Logging;
 
@@ -98,6 +99,9 @@ public static async Task Run(JobBase job, string[] commandLineArgs)
             {
                 _logger.LogError("Job runner threw an exception: {Exception}", ex);
             }
+
+            Trace.Close();
+            TelemetryConfiguration.Active.TelemetryChannel.Flush();
         }
 
         private static ILoggerFactory ConfigureLogging(JobBase job)