[APM][Tracing] Add documentation for DD_TRACE_OTEL_ENABLED and related integrations

[pierotibou]

What does this PR do? What is the motivation?

Documents DD_TRACE_OTEL_ENABLED. It also starts listing integrations that we explicitly test with this setup.
I've chosen to list those in a separated table to better explain the need to set DD_TRACE_OTEL_ENABLED to true. But maybe one table with an extra column would be better?

Merge instructions

• Please merge after reviewing

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/pierre/otel/tracing/trace_collection/compatibility/dotnet-core
• https://docs-staging.datadoghq.com/pierre/otel/tracing/trace_collection/compatibility/dotnet-framework
• https://docs-staging.datadoghq.com/pierre/otel/tracing/trace_collection/library_config/dotnet-core
• https://docs-staging.datadoghq.com/pierre/otel/tracing/trace_collection/library_config/dotnet-framework

[zacharycmontoya]

Hey Pierre, the application also needs to add a setting so that the Azure SDK will generate OpenTelemetry spans to begin with: https://devblogs.microsoft.com/azure-sdk/introducing-experimental-opentelemetry-support-in-the-azure-sdk-for-net/

[zacharycmontoya]

For reference, in our .NET Tracer samples we use the AppContext switch: https://github.com/DataDog/dd-trace-dotnet/blob/ee0f88c47524df58112778e19fb393e57206a822/tracer/test/test-applications/integrations/Samples.AzureServiceBus/Program.cs#L24

[cswatt]

Added DOCS-6901 to review.

[zacharycmontoya]

The .NET content looks good to me

[brett0000FF]

Thanks for adding this content! Please see a few of my suggestions and questions for consideration. 🙏

[brett0000FF]

Suggested change

	Some libraries provide built in [Activity based tracing][13]. This is the same mechanism the OpenTelemetry project relies on. By setting `DD_TRACE_OTEL_ENABLED` to `true`, the .NET tracer will automatically resurface traces provided by the libraries themselves. This is possible since [version 2.21.0][4]. Here are a list of libraries that are tested with this setup (more libraries provide such tracing though, they aren't yet expliciitly tested).
	Some libraries provide built-in [Activity based tracing][13]. This is the same mechanism that OpenTelemetry is based on.
	For these libraries, set `DD_TRACE_OTEL_ENABLED` to `true`, and the .NET tracer automatically captures traces their traces. This is supported since [version 2.21.0][4].
	The following list of libraries have been tested with this setup:

Is the distinction about Activity-based tracing important, or could we simply say "Some libraries provide built-in support for OpenTelemetry"?

[pierotibou]

I assume it's not that important. we could maybe switch to talking about OpenTelemetry only. @zacharycmontoya will know better though

[pierotibou]

Thanks a lot @brett0000FF, I had completely missed your review and just realized today this hadn't been merged.

[pierotibou]

Thanks again @brett0000FF !