Document built-in System.Net activities

[antonfirsov]

.NET 9 introduced additions to System.Net distributed tracing which define a contract following OTel recommendations:

• Add standard tags to HttpClient native trace instrumentation runtime#104251 adjusted the HTTP client request activity so it natively complies with OTel recommendations. This included adding standard tags.
• Activities for Http Connections, Dns, Sockets and SslStream runtime#103922 introduced activities breaking down the HTTP connection setup.

This PR documents the activities together with their attributes in a manner similar to the metrics docs added by #37213. The matching semconv PR is open-telemetry/semantic-conventions#1192. Conceptual docs are being introduced in #42830.

Moreover, the PR also adjusts certain parts of the metrics doc in order to make it more accurate, synchronize it with the activities doc and incorporate the changes from #42810.

---

Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/built-in-metrics-system-net.md	System.Net metrics
docs/core/diagnostics/distributed-tracing-builtin-activities.md	Built-in Activities in .NET
docs/core/diagnostics/distributed-tracing-concepts.md	.NET distributed tracing concepts
docs/core/diagnostics/distributed-tracing.md	docs/core/diagnostics/distributed-tracing
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc

[tarekgh]

LGTM. Maybe @MihaZupan can help looking too to validate the details.

[MihaZupan]

This is great, thank you!