Description

Fixes the incorrect implementation of the dotnet.thread_pool.thread.count runtime metric. The metric was implemented as a monotonic Counter using CreateObservableCounter, but according to the official documentation, it should be an UpDownCounter since it represents "the number of thread pool threads that currently exist" - a value that can both increase and decrease over time.

Changes

Changed the metric creation from:

s_meter.CreateObservableCounter(
    "dotnet.thread_pool.thread.count",
    () => (long)ThreadPool.ThreadCount,
    unit: "{thread}",
    description: "The number of thread pool threads that currently exist.");

To:

s_meter.CreateObservableUpDownCounter(
    "dotnet.thread_pool.thread.count",
    () => (long)ThreadPool.ThreadCount,
    unit: "{thread}",
    description: "The number of thread pool threads that currently exist.");

This aligns the implementation with:

• The official Microsoft documentation
• The semantic meaning of the metric (thread count can go up or down)
• Other similar metrics in the codebase like dotnet.timer.count and dotnet.assembly.count

Testing

• All 397 existing unit tests pass, including RuntimeMetricsTests
• Manual verification confirms the metric now correctly reports as ObservableUpDownCounter<System.Int64>
• No breaking changes to the metric name or data - only the instrument type changes

Fixes #110757

Fixes #112077

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