Document Tracing/Activity sampling

There is infrastructure via ActivityListener.Sample and in ASP.NET core when the request comes in to pull the trace flags from the incoming request to create the TraceID and set the Recorded bit on the activity. These can be overridden if necessary to add custom logic such as only trusting trace information from certain hosts, however its not currently documented.

@noahfalk has the knowledge in his head - we need to put this into documentation in the form of a sampling scenarios HowTo or cookbook.