Distributed tracing in networking

[antonfirsov]

This is a major rework of #42830, extending the content, making the information more specific and IMO easier to follow. The biggest change is the in the collection paragraphs:

• I dropped the definitions of terms instrumentation, collection, export, OpenTelemetry etc., given that we were already using them in the intro, so it felt rather pointless to define them at that point; IMO the reader is looking for practical guidelines and examples anyways.
• Instead, I added a summary of the available methods, so readers can navigate the document following their preferences and goals.
• Metrics.md: Added back the dotnet-monitor example. Reintroduced a short paragraph on prometheus/grafana, since the referenced docs need some augmentation.

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Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/distributed-tracing-builtin-activities.md	Built-in activities in .NET
docs/fundamentals/networking/telemetry/metrics.md	Networking metrics
docs/fundamentals/networking/telemetry/overview.md	Networking telemetry in .NET
docs/fundamentals/networking/telemetry/tracing.md	docs/fundamentals/networking/telemetry/tracing
docs/fundamentals/toc.yml	docs/fundamentals/toc

[samsp-msft]

Great content. Lets make the changes to the core telemetry example so you don't need to document how to modify it.

[antonfirsov]

@gewarren would you mind doing a language review on this? (Though I think it's mostly ok, so it's optional.)

[gewarren]

I'll wait to review the rest until you've run Acrolinx (see one of the comments I left).

[antonfirsov]

@gewarren I pushed the Acrolinx scores into the 80-90 zone in 17ea9e2.
There were many recommendations which were either very difficult to address or would end up in loss of nuanced information/context. I did not address those.

[gewarren]

I didn't finish reviewing tracing.md, as it seems to duplicate a lot of the content in metrics.md. I'm guessing that's not intentional, so will hold off on reviewing the rest.

[antonfirsov]

@gewarren thanks a lot for chiming in, the content looks much better now!