Support new DiagnosticSource.IsEnabled(string, object, object) API

[lmolkova]

There were couple of API recently introduced in dotnet/corefx#15984 and dotnet/corefx#15985:

• DiagnosticSource.IsEnabled(string, object,object)
• DiagnosticListener.Subscribe(IObserver<KeyValuePair<string, object>> observer, Func<string, object, object, bool> isEnabled)

DiagnosticListenerExtensions.SubscribeWithAdapter and DiagnosticSourceAdapter constructors should provide an overloads to subscribe with Func<string, object, object, bool>

[Eilon]

cc @rynowak FYI

[Eilon]

@rynowak please chime in if you have any concerns, but this seems like a good idea to me.

[rynowak]

Yep. This seems like something we need to do. I don't have any idea yet what the scope of the change is.

[lmolkova]

👍