add os platform

Author: NoriZC

shell/agents/Microsoft.Azure.Agent/Telemetry.cs

@@ -231,6 +231,7 @@ private void LogTelemetry(AzTrace trace, Exception exception)
             ["EventType"] = trace.EventType,
             ["ShellCommand"] = trace.ShellCommand,
             ["Details"] = GetDetailedMessage(trace.Details),
+            ["OSPlatform"] = AzTrace.Platform
         };
 
         if (exception is null)