Net core 2.0 HttpClient will start to send new and deprecated events in the next release

[lmolkova]

Currently HttpClient on .net core 2.0 check if someone listens to 'System.Net.Http.HttpRequestOut*' event and then send 'new' evenst only.
Otherwise, for backward compatibility it also checks if someone listens to deprecated 'System.Net.Http.Request/Response' events.

This introduces issue described in https://github.com/dotnet/corefx/issues/18762 when 2 different listeners are interested in different events: e.g. Glimpse of old version and AI of new version.

In the future release of HttpClient, it will check for both events and will send both if they are enabled: dotnet/corefx#19659

HttpCoreDiagnosticSourceListener listens to both sets of events (because it targets netstandard1.6 and does not know which runtime will be used). So with the future HttpClient, it will report the same telemetry twice.

AI Listener needs to ignore 'old' event on the new runtime similarly to how it's done in AspNetCore incoming request listener, but old events should be rejected in IsEnabled callback, rather than in the event callback.

This needs to be done in 2.4.0 stable release