Add built-in OpenTelemetry metrics to MemoryCache

[rjmurillo]

Summary

Adds OpenTelemetry metrics to MemoryCache. Follows the System.Net.Http pattern (#87319).

Fixes #124140

API Changes

Microsoft.Extensions.Caching.Abstractions

public class MemoryCacheStatistics
{
    public long TotalEvictions { get; init; } // NEW
}

Microsoft.Extensions.Caching.Memory

public class MemoryCacheOptions
{
    public string Name { get; set; } = "Default"; // NEW
}

public class MemoryCache
{
    // ILoggerFactory now nullable
    public MemoryCache(IOptions<MemoryCacheOptions> optionsAccessor, ILoggerFactory? loggerFactory);

    // NEW
    public MemoryCache(IOptions<MemoryCacheOptions> optionsAccessor, ILoggerFactory? loggerFactory, IMeterFactory? meterFactory);
}

Instruments

Meter: Microsoft.Extensions.Caching.Memory.MemoryCache

Instrument	Type	Dimensions
cache.requests	ObservableCounter<long>	cache.name, cache.request.type (hit/miss)
cache.evictions	ObservableCounter<long>	cache.name
cache.entries	ObservableUpDownCounter<long>	cache.name
cache.estimated_size	ObservableGauge<long>	cache.name

Design

• Gated on TrackStatistics — no instruments created unless TrackStatistics = true
• SharedMeter fallback — no IMeterFactory, no problem. Singleton SharedMeter with NOP Dispose. Same pattern as System.Net.Http.MetricsHandler.SharedMeter
• MeterOptions tags — factory path sets Tags = [new("cache.name", _options.Name)] for meter deduplication in DefaultMeterFactory
• WeakReference callbacks — observable callbacks capture WeakReference<MemoryCache>. Dead caches return empty measurements. No GC leaks
• IEnumerable overloads — Func<IEnumerable<Measurement<long>>> prevents phantom zero measurements from disposed or collected caches
• Eviction counting — RemoveEntry calls ConcurrentDictionary.TryRemove(KVP). Only the thread that removes the entry increments TotalEvictions. No double-counting
• DI unchanged — new constructor auto-selected. No changes to MemoryCacheServiceCollectionExtensions
• Conditional DiagnosticSource ref — in-box for net11.0+

Tests

• 142 tests pass (net11.0)
• 11 new metrics tests: instrument creation, SharedMeter fallback, null factory/logger, TrackStatistics gate, Name property, eviction accuracy, disposed cache, MeterOptions tags, over-count prevention
• 2 new eviction statistics tests

[dotnet-policy-service]

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[Copilot]

Pull request overview

Adds built-in OpenTelemetry metrics support to Microsoft.Extensions.Caching.Memory.MemoryCache by exposing additional cache statistics and wiring them into observable instruments, gated by MemoryCacheOptions.TrackStatistics.

Changes:

• Added TotalEvictions to MemoryCacheStatistics and tracked eviction counts in MemoryCache.
• Introduced MemoryCacheOptions.Name and new MemoryCache ctor overload supporting IMeterFactory.
• Added unit tests for metrics publication and eviction statistics, plus conditional DiagnosticSource references for non-inbox TFMs.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/libraries/Microsoft.Extensions.Caching.Memory/src/MemoryCache.cs	Implements eviction counting and publishes OTEL observable instruments via Meter/IMeterFactory.
src/libraries/Microsoft.Extensions.Caching.Memory/src/MemoryCacheOptions.cs	Adds Name option used as a metrics dimension.
src/libraries/Microsoft.Extensions.Caching.Memory/src/Microsoft.Extensions.Caching.Memory.csproj	Adds conditional DiagnosticSource reference for TFMs missing in-box metrics types.
src/libraries/Microsoft.Extensions.Caching.Memory/ref/Microsoft.Extensions.Caching.Memory.cs	Updates public API surface for new ctor overload and Name option.
src/libraries/Microsoft.Extensions.Caching.Memory/tests/MemoryCacheMetricsTests.cs	Adds tests validating instrument creation and basic measurements.
src/libraries/Microsoft.Extensions.Caching.Memory/tests/MemoryCacheGetCurrentStatisticsTests.cs	Adds tests for eviction statistics behavior.
src/libraries/Microsoft.Extensions.Caching.Memory/tests/Microsoft.Extensions.Caching.Memory.Tests.csproj	Adds conditional DiagnosticSource project reference for .NETFramework tests.
src/libraries/Microsoft.Extensions.Caching.Abstractions/src/MemoryCacheStatistics.cs	Adds TotalEvictions to the statistics snapshot type.
src/libraries/Microsoft.Extensions.Caching.Abstractions/ref/Microsoft.Extensions.Caching.Abstractions.cs	Updates ref assembly for TotalEvictions.

[cincuranet]

For the _accumulatedEvictions comments from Copilot, I think the idea with bool return is sound.

Or maybe putting the logic into RemoveEntry - given there's already _cacheSize handling - with a flag whether to count or not (not all RemoveEntry calls (should) update _accumulatedEvictions).

Up to you.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

[ManickaP]

LGTM