Code issue on article about OpenTelemetry and Application Insights

### Type of issue

Code doesn't work

### Description

There is a small issue in the article which prevents the sample code from working.
In step two the following code is added:
```csharp
if (!string.IsNullOrEmpty(builder.Configuration["APPLICATIONINSIGHTS_CONNECTION_STRING"]))
{
otel.UseAzureMonitor();
}
```
In step three it's recommended to add the following json to `appsettings.json`:
```json
"AzureMonitor": {
"ConnectionString": "InstrumentationKey=12345678-abcd-abcd-abcd-12345678..."
}
```

Because of this `otel.UseAzureMonitor();` is never called.
An easy fix would be to update code in step 2 as so:
```csharp
if (!string.IsNullOrEmpty(builder.Configuration["APPLICATIONINSIGHTS_CONNECTION_STRING"] ?? builder.Configuration["AzureMonitor:ConnectionString"]))
{
otel.UseAzureMonitor();
}
```
or something similar.

Thank you!

### Page URL

https://learn.microsoft.com/en-us/dotnet/core/diagnostics/observability-applicationinsights

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/core/diagnostics/observability-applicationinsights.md

### Document Version Independent Id

e14803f7-d5c7-657a-763c-680cf4197365

### Article author

@tommcdon

### Metadata

* ID: 45cce027-ad78-e3d1-7144-6091a9b5f023
* Service: **dotnet-fundamentals**