Investigate DiagnosticsSource for protocol diagnostics

[zivkan]

System.Net.Http.HttpClient emits events though DiagnosticSource: https://github.com/dotnet/corefx/blob/master/src/System.Net.Http/src/HttpDiagnosticsGuide.md

Note, it works on both .NET Framework and .NET Core, but the event names are different (probably because the event datastructure is different), so if we use it, it's important to test both runtimes. Also, check what happens on mono.

This will let us get much more detailed information, if we want it, but depending on what we want, it might also not give us the level of information we want.

[zivkan]

From my investigations, it's not a good fit. One of the only advantages it has is that it's a single place to receive request completion and request error notifications. However, the differences between how the .NET Framework and .NET Core versions work increase the complexity, particularly when the request failed. It's simpler to use an exception handler, as HttpRetryHandler already does.