Add source generated metrics doc

[mariamgerges]

Summary

This pull request includes updates to the documentation for .NET metrics, specifically focusing on the introduction and usage of source-generated metrics and the addition of installation instructions for the related NuGet package. The most important changes include adding a new documentation file for source-generated metrics, updating existing documentation to reference this new file, and providing detailed usage examples.

Documentation updates:

• docs/core/diagnostics/metrics-strongly-typed.md: Added instructions for installing the Microsoft.Extensions.Telemetry.Abstractions NuGet package and included a new usage example with necessary using statements. [1] [2]
• docs/core/diagnostics/source-generated-metrics.md: Created a new file explaining how to use source-generated metrics in .NET, including installation instructions, usage examples, and requirements for metric methods.

References and navigation updates:

• docs/core/diagnostics/metrics-strongly-typed.md: Added a "See also" link to the new source-generated metrics documentation.
• docs/core/diagnostics/metrics.md: Added a reference to the new source-generated metrics documentation.
• docs/navigate/tools-diagnostics/toc.yml: Updated the table of contents to include links to the new source-generated metrics documentation and the strongly-typed metrics documentation.
  Fixes #Issue_Number (if available)

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Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/metrics-generator.md	docs/core/diagnostics/metrics-generator
docs/core/diagnostics/metrics-strongly-typed.md	docs/core/diagnostics/metrics-strongly-typed
docs/core/diagnostics/metrics.md	docs/core/diagnostics/metrics
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc

[dariusclay]

What's the main difference between this update and https://learn.microsoft.com/dotnet/core/diagnostics/metrics-strongly-typed ?

[mariamgerges]

What's the main difference between this update and https://learn.microsoft.com/dotnet/core/diagnostics/metrics-strongly-typed ?

this is generally for fast metering, while the other is for metrics with strong types.

[IEvangelist]

This is a great start, just a few nits. One thing we should also do, is move all the code to under a snippets directory. So, under the docs/core/diagnostics folder, you'll create a snippets folder - if there isn't one already. You'll add a new folder specific to the topic, maybe metrics - and then put the solution, projects, and code files there.

Then back in the Markdown file, you reference the code using the following syntax:

:::code language="csharp" source="snippets/metrics/Path/To/Code.cs":::

[mariamgerges]

This is a great start, just a few nits. One thing we should also do, is move all the code to under a snippets directory. So, under the docs/core/diagnostics folder, you'll create a snippets folder - if there isn't one already. You'll add a new folder specific to the topic, maybe metrics - and then put the solution, projects, and code files there.

Then back in the Markdown file, you reference the code using the following syntax:

:::code language="csharp" source="snippets/metrics/Path/To/Code.cs":::

I updated all the code snippets, only one left which shows how the generated code will look like, so I couldn't update it.