Remove magic strings from documentation

### Type of issue

Typo

### Description

In the code for section 5, Configure OpenTelemetry with the Correct Providers, the following code sample is given:

```C#
// Metrics provider from OpenTelemetry
.AddAspNetCoreInstrumentation()
.AddMeter(greeterMeter.Name)
// Metrics provides by ASP.NET Core in .NET 8
.AddMeter("Microsoft.AspNetCore.Hosting")
.AddMeter("Microsoft.AspNetCore.Server.Kestrel")
// Metrics provided by System.Net libraries
.AddMeter("System.Net.Http")
.AddMeter("System.Net.NameResolution")
.AddPrometheusExporter()
```

The calls to
```C#
.AddMeter("Microsoft.AspNetCore.Hosting")
.AddMeter("Microsoft.AspNetCore.Server.Kestrel")
```
are not necessary because they are already included with the call ```.AddAspNetCoreInstrumentation()```

Additionally, the calls to
```C#
.AddMeter("System.Net.Http")
.AddMeter("System.Net.NameResolution")
```
should be replaced with a call to ```.AddHttpClientInstrumentation()```

Using magic strings in documentation examples is confusing and should be discouraged - it makes me think that there are more undocumented ones that I'm missing out on, or that by calling the existing extensions, like ```.AddAspNetCoreInstrumentation()``` I'm not getting the hosting/kestrel meters, which is wrong.

### Page URL

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

### Content source URL

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

### Document Version Independent Id

59d4807e-8b88-9d39-cd51-977792064cf6

### Article author

@tommcdon

### Metadata

* ID: eded9b69-1a38-a8bb-7e76-2f72a2e78abe
* PlatformId: 2f326b35-4a98-0694-da18-c1eb7c9dd0eb
* Service: **dotnet-fundamentals**

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