System.Net.Dns.RunAsync() does not decrement telemetry for canceled requests

[sandersaares]

If a call that ends up in Dns.RunAsync() is canceled, nothing ever calls NameResolutionTelemetry.AfterResolution() and so the variable NameResolutionTelemetry._currentLookups remains forever incremented by one.

The unmatched BeforeResolution() is called in Dns.RunAsync() here: https://github.com/dotnet/runtime/blob/9101b853a1f5194de3e7bc495454a57df0da300e/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs#L652C16-L652C16

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Issue Details

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If a call to Dns.GetHostAddressesAsync() (or similar) is canceled, nothing ever calls NameResolutionTelemetry.AfterResolution() and so the variable NameResolutionTelemetry._currentLookups remains forever incremented by one.

The unmatched BeforeResolution() is called in Dns.RunAsync() here: https://github.com/dotnet/runtime/blob/9101b853a1f5194de3e7bc495454a57df0da300e/src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs#L652C16-L652C16

Author:	sandersaares
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[karelz]

Triage: Worth having, but not critical, moving to 9.0.
We should consider including it in servicing of #81023 if we come to that.