[minor] Improve DependencyCollectorEventSource.Log.CurrentActivityIsNull event

[lmolkova]

Internal customer noticed some suspicious traces in their telemetry

AI (Internal): Current Activity is null

This is DepedencyCollector event source event fired by HttpCoreDiagnosticSourceListener

when the current activity is null.

There were not that many occurrences of this error (30 out of thousands reported HTTP dependency calls over 24 hours).

Based on the event. it's not possible to determine where it happened and whether it indicates a bug - there are valid cases when it may happen.

Even should include DiagnosticSource event where the error happened to be able to say whether it's a bug causing it or it's a valid case.

Version Info

SDK Version : 2.4+
.NET Version : .NEt core 2.0
How Application was onboarded with SDK(VisualStudio/StatusMonitor/Azure Extension) :
OS : unknown, does not matter
Hosting Info (IIS/Azure WebApps/ etc) : does not matter