Support W3C format in System.Net for distributed trace context propagation

[SergeyKanzhelev]

Motivation

Support extraction and injection of distributed tracing context in a format of W3C headers for System.Net alongside Request-Id.

This issue depends on tracestate collecton implementation in Activity class.

CC: @seth-capistron, @vancem, @jacpull, @lmolkova, @glennc, @davidfowl

[karelz]

@SergeyKanzhelev what do you expect will need to happen in System.Net for that? New APIs?

[lmolkova]

@karelz

@SergeyKanzhelev means (correct me if I'm wrong) that HttpClient diagnostics instrumentation should be changed to support new standard:

https://github.com/dotnet/corefx/blob/master/src/System.Net.Http/src/System/Net/Http/DiagnosticsHandler.cs#L82

It should not involve any new APIs creation.

[davidsh]

These PRs dotnet/corefx#35880, dotnet/corefx#35882 address this issue.

cc: @lmolkova