Add a reference for new .NET 8 metrics

[noahfalk]

Part of the work for #36748

This is part of new metrics reference material. If the patterns look good I can add the rest, but also happy to shuffle things around. For now I put all the metrics into files within the diagnostics folder, but I can move them anywhere area owners would like them to be. In the future I imagine adding more pages for runtime metrics, Microsoft.Extensions metrics, EF metrics, and potentially any other area of the runtime that adds metrics. The overview admittedly feels a little spartan for now.

@JamesNK @antonfirsov @tarekgh @samsp-msft

---

Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/available-counters.md	Well-known EventCounters in .NET
docs/core/diagnostics/built-in-metrics-aspnetcore.md	ASP.NET Core Metrics
docs/core/diagnostics/built-in-metrics-system-net.md	System.Net Metrics
docs/core/diagnostics/built-in-metrics.md	Built-in Metrics in .NET
docs/core/diagnostics/compare-metric-apis.md	docs/core/diagnostics/compare-metric-apis
docs/core/diagnostics/metrics-collection.md	Collect metrics
docs/core/diagnostics/metrics.md	Metrics Overview

[tarekgh]

The doc structure and template look good to me. I left a couple of questions.

[JamesNK]

Overall, looks good. Needs a thorough edit pass.

[noahfalk]

Thanks for feedback so far! I added the remaining ASP.NET metrics, updated for feedback, and hopefully caught some of the editing issues.

[antonfirsov]

Apologies for the late review. Blocking merge since there is one major inaccuracy.

[antonfirsov]

We need to sync this with the coming System.Net changes, probably easier to do it right in the PR.

[Rick-Anderson]

Overall, looks good. Needs a thorough edit pass.

If the dotnet doc folks can't do this, @tdykstra and I can help. See open-telemetry/semantic-conventions#283 where @tdykstra and I have made massive edits.

[Rick-Anderson]

@noahfalk throwing out this idea. It would be much more productive for you, me, and @tdykstra if you merged this PR and we had 24 hours to do an edit PR.

I'd rather PR this PR but I don't have permission.

[noahfalk]

It would be much more productive for you, me, and @tdykstra if you merged this PR and we had 24 hours to do an edit PR.

Done :)