Regression issue: appId is never set in response headers

[lmolkova]

Repro Steps

1. create a new aspnet core app from the template
2. send some requests

Actual Behavior

app always returns Request-Context: appId= (empty) header

Expected Behavior

non-empty appid is returned

Version Info

It was introduced in 2.8.0-beta1 with perf fixes:

In the below code lastIKeyLookedUp is always null and is never equal to requestTelemetry.Context.InstrumentationKey so lastAppIdUsed never gets a chance to be anything except null.

ApplicationInsights-aspnetcore/src/Microsoft.ApplicationInsights.AspNetCore/DiagnosticListeners/Implementation/HostingDiagnosticListener.cs

Lines 525 to 532 in a14386c

	if (this.lastIKeyLookedUp != requestTelemetry.Context.InstrumentationKey)
	{
	this.lastIKeyLookedUp = requestTelemetry.Context.InstrumentationKey;
	this.applicationIdProvider?.TryGetApplicationId(requestTelemetry.Context.InstrumentationKey, out this.lastAppIdUsed);
	}
	HttpHeadersUtilities.SetRequestContextKeyValue(responseHeaders,
	RequestResponseHeaders.RequestContextTargetKey, this.lastAppIdUsed);

[cijothomas]

when lastIKeyLookedUp does not match the current items ikey, we trigger a app id lookup, and stores ikey to lastIKeyLookedUp. I believe the issue is relying incorrectly on ApplicationIdProvider to resolve ikey to appid in the very 1st call. its not guaranteed to return app id for the 1st time (because it don't wait for task to complete), but gets it resolved by subsequent calls. The above perf fix prevented asking for appid once lastIKeyLookedUp is set.

This is an issue with actual AppIdProvider, not with a DictionaryProvider. Functional Tests used DictionaryProvider, hence did not catch the issue.

The fix would be to do look up when lastAppIdUsed is null.

[cijothomas]

if (this.lastIKeyLookedUp != requestTelemetry.Context.InstrumentationKey)
                    {
                        var appIdResolved = this.applicationIdProvider?.TryGetApplicationId(requestTelemetry.Context.InstrumentationKey, out this.lastAppIdUsed);
                        if (appIdResolved.HasValue && appIdResolved.Value)
                        {
                            this.lastIKeyLookedUp = requestTelemetry.Context.InstrumentationKey;
                        }
                    }