APPINSIGHTS_INSTRUMENTATIONKEY AppSetting overrides ikey specified in code

[AlexBulankou]

1. Create 4.6 Web Application, enable Application Insights SDK 2.2 Beta 4 and specify in Global.asax.cs:

2. TelemetryConfiguration.Active.InstrumentationKey = "ikey1";

3. Publish to Azure WebApp and verify that telemetry is being sent to ikey1

4. Add AppSetting to WebApp: APPINSIGHTS_INSTRUMENTATIONKEY pointing to ikey2

Expected: telemetry continues to be sent to ikey1, because settings in code take priority
Actual: telemetry now is being sent to ikey2, while QuickPulse events continue to go to ikey1.

[SergeyKanzhelev]

Since I took a look at code anyway - there are two places we set the key from the env variable:

This will not respect TelemetryConfiguration.Active.InstrumentationKey = "ikey1";, but will work when you set new TelemetryClient("ikey1").TrackTrace("trace"):

https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/src/Core/Managed/Shared/TelemetryClient.cs#L382-L387

This logic will not "override" the setting from the configuration file, but TelemetryConfiguration.Active.InstrumentationKey = "ikey1";, will override it:

https://github.com/Microsoft/ApplicationInsights-dotnet/blob/de488729c28b721a758e0024ea2d4f79909ff797/src/Core/Managed/Net40/Extensibility/Implementation/TelemetryConfigurationFactory.cs#L76-L81

So clearly the code needs to be refactored and we need unit tests that will "describe" the expected behavior

[dnduffy]

There is one small problem here, how do you tell the difference between a value in TelemetryConfiguration.Active.InstrumentationKey that was set explicitly in code from one that TelemetryConfigurationFactory.Instance.Initialize placed there from reading the config entry?

[dnduffy]

I've changed the logic and updated the unit tests.
Pull request is here: microsoft/ApplicationInsights-dotnet#379