DependencyCollector for .NET Core: memory leak on netcoreapp1.1 and prior

[lmolkova]

DependencyCollector keeps pending telemetry list.
In BeginRequest event it adds telemetry to the concurrent dictionary
In EndRequest event it removes telemetry from the list.

However on netcoreapp1.1 and prior, EndRequest is not guaranteed. It's not fired for canceled and faulted (DNS, network issues, etc..) requests.

In every long running service this list will always grow. In case of massive network issues service may die because of OOM.