Need some telemetry initializer to populate node_name and role_instance name as the base sdk implementation does not work in Linux

[cijothomas]

There is NO nodename or role_instance properties set when app is running in Ubuntu with 2.3.0-beta2 onwards.

If not populated already, base SDK puts this.GetEnvironmentVariable("COMPUTERNAME"); as node name and roleinstance. However, in Ubuntu this environment variable is null and hence both properties remain null.
Livemetrics falls back to use Environment.MachineName which seem to work in Linux as well.

This PR (https://github.com/Microsoft/ApplicationInsights-aspnetcore/pull/656/files) removed the deprecated TelemetryInitializer which used to populate the same with Dns.GetHostName(), which seem to be working in linux as well.

We should probably write a new TelemetryInitializers for asp.net core which either uses Environment.MachineName or Dns.GetHostName()

Version Info

SDK Version : 2.3.0-beta2
.NET Version :
How Application was onboarded with SDK(VisualStudio/StatusMonitor/Azure Extension) :
OS :
Hosting Info (IIS/Azure WebApps/ etc) :

[cijothomas]

Fixed #672