Generate spans for remote calls invoked via System.Net.WebRequest (or sub-classes)

Currently we are unable to instrument any http or https requests in asp.net. For eg: when we do `Response.Redirect` or 

```csharp
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(txtinput.Text);
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
```

We are not able to instrument this `HttpRequest` containing `txtinput.Text`.