`APPINSIGHTS_INSTRUMENTATIONKEY` interfers with multi-dimensional telemetry

#### Check for a solution in the Azure portal
For issues in production, please check for a solution to common issues in the Azure portal before opening a bug. In the Azure portal, navigate to your function app, select `Diagnose and solve problems` from the left, and view relevant dashboards before opening your issue.

#### Investigative information

Please provide the following:

- Timestamp: N/A
- Function App version: `~3`
- Function App name: `azure-autoscaling-toolbox` (works) & `azure-autoscaling-toolbox-repro` (does not work)
- Function name(s) (as appropriate): `kubernetes-app-scaled-event`
- Invocation ID: N/A
- Region: West Europe

#### Repro steps

Provide the steps required to reproduce the problem:

I have deployed the same application on 2 Function Apps:
1. Deploy Function App
2. Configure Application Insights key
3. Call endpoint
4. Write metric with multiple dimensions (through [Arcus Observability](https://observability.arcus-azure.net/), not the built-in LogMetric extension, see https://github.com/tomkerkhove/azure-autoscaling-toolbox/blob/main/samples/autoscaling-over-time-dashboard/src/AzureAutoscalingToolbox.Samples.StatefulAppInstances/Entities/ApplicationEntity.cs#L71)

#### Expected behavior

Provide consistent telemetry

#### Actual behavior

Custom dimensions are ommitted through some voodoo.

- Left: `APPLICATIONINSIGHTS_INSTRUMENTATIONKEY` configuration key name
- Right: `APPINSIGHTS_INSTRUMENTATIONKEY` configuration key name

#### Known workarounds

Use another configuration key than `APPINSIGHTS_INSTRUMENTATIONKEY`.

#### Related information

Provide any related information

* Programming language used: .NET
* Links to source: https://github.com/tomkerkhove/azure-autoscaling-toolbox/tree/main/samples/autoscaling-over-time-dashboard
* Bindings used: HTTP

Start with the referenced ApplicationEntity.cs example and reproduce the HTTP function behavior in the two Function Apps using the two instrumentation-key configuration names. Compare the resulting multi-dimensional telemetry and trace how the Function App v3 host handles those settings; done means custom dimensions are consistent with either supported key name.

Written by the indexing model from the issue text.