Fix dotnet.thread_pool.thread.count metric to use UpDownCounter instead of Counter

[Copilot]

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

Original prompt

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This section details on the original issue you should resolve

<issue_title>Runtime metrics for ThreadPool ThreadCount is not implemented as UpDownCounter</issue_title>
<issue_description>In the documentation it is mentioned that the ThreadPool ThreadCount metics is of type UpDownCounter

But it looks like it is implemented as a Monotonic counter as it uses CreateObservableCounter instead of CreateObservableUpDownCounter

runtime/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/RuntimeMetrics.cs

Line 99 in 6f3f675

"dotnet.thread_pool.thread.count",

Shouldn't it be an UpDownCounter or a Gauge if it is The number of thread pool threads that currently exist?</issue_description>

Comments on the Issue (you are @copilot in this section)

@tommcdon @noahfalk @tarekgh @noahfalk Yeah, that looks like a bug. Thanks for the report @rakeshv1! cc @samsp-msft

@tarekgh - mind if I assign this to you?</comment_new>
<comment_new>@tarekgh
I assigned it to myself. @rakeshv1 are you interested in submitting a PR for that? </comment_new>

Fixes #112077

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[Copilot]

Pull Request Overview

This PR fixes a bug in the dotnet.thread_pool.thread.count runtime metric implementation. The metric was incorrectly using a monotonic Counter (which only increases) instead of an UpDownCounter (which can increase or decrease), which does not match the semantic meaning of thread count nor the official documentation.

Key Changes:

• Changed CreateObservableCounter to CreateObservableUpDownCounter for the thread pool thread count metric
• Aligns implementation with Microsoft's official documentation and similar metrics in the codebase

[tarekgh]

CC @noahfalk to ensure awareness.

[tarekgh]

/ba-g build failures are not related