Breaking DiagnosticSource changes for Http on NET 4.6

[lmolkova]

Heads up on DiagnosticSource change in PR 18502.

This change affects reflection hook that instruments WebRequests on .NET46 with DiganosticSource.
It changes events names and starts Activity/injects headers itself.

The rationale for this change: events sent with this hook should be consistent with HttpClient instrumentation on .NET Core. If we don't do it right now, before it became public, we would have to deal with deprecation hell later and we know it's going to be painful for everyone: event producer and consumers (including AI).

What needs to be fixed in AI 2.4.0 Web SDK:

• event names
• AI should not start Activity for dependency if the hook was called.