Address the feedback

src/libraries/Common/src/System/Diagnostics/DiagnosticsHelper.cs

@@ -0,0 +1,68 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+
+namespace System.Diagnostics
+{
+    internal static class DiagnosticsHelper
+    {
+        internal static bool CompareTags(IEnumerable<KeyValuePair<string, object?>>? tags1, IEnumerable<KeyValuePair<string, object?>>? tags2)
+        {
+            if (tags1 is null)
+            {
+                return tags2 is null;
+            }
+
+            if (tags2 is null)
+            {
+                return false;
+            }
+
+            if (tags1 is ICollection<KeyValuePair<string, object?>> firstCol && tags2 is ICollection<KeyValuePair<string, object?>> secondCol)
+            {
+                int count = firstCol.Count;
+                if (count != secondCol.Count)
+                {
+                    return false;
+                }
+
+                if (firstCol is IList<KeyValuePair<string, object?>> firstList && secondCol is IList<KeyValuePair<string, object?>> secondList)
+                {
+                    for (int i = 0; i < count; i++)
+                    {
+                        KeyValuePair<string, object?> pair1 = firstList[i];
+                        KeyValuePair<string, object?> pair2 = secondList[i];
+                        if (pair1.Key != pair2.Key || !object.Equals(pair1.Value, pair2.Value))
+                        {
+                            return false;
+                        }
+                    }
+
+                    return true;
+                }
+            }
+
+            using (IEnumerator<KeyValuePair<string, object?>> e1 = tags1.GetEnumerator())
+            using (IEnumerator<KeyValuePair<string, object?>> e2 = tags2.GetEnumerator())
+            {
+                while (e1.MoveNext())
+                {
+                    KeyValuePair<string, object?> pair1 = e1.Current;
+                    if (!e2.MoveNext())
+                    {
+                        return false;
+                    }
+
+                    KeyValuePair<string, object?> pair2 = e2.Current;
+                    if (pair1.Key != pair2.Key || !object.Equals(pair1.Value, pair2.Value))
+                    {
+                        return false;
+                    }
+                }
+
+                return !e2.MoveNext();
+            }
+        }
+    }
+}

src/libraries/Microsoft.Extensions.Diagnostics.Abstractions/ref/Microsoft.Extensions.Diagnostics.Abstractions.cs

@@ -1,7 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-namespace Microsoft.Extensions.Metrics
+namespace Microsoft.Extensions.Diagnostics.Metrics
 {
     public interface IMeterFactory : System.IDisposable
     {

src/libraries/Microsoft.Extensions.Diagnostics.Abstractions/ref/Microsoft.Extensions.Diagnostics.Abstractions.csproj

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <CLSCompliant>false</CLSCompliant>

src/libraries/Microsoft.Extensions.Diagnostics.Abstractions/src/IMeterFactory.cs → src/libraries/Microsoft.Extensions.Diagnostics.Abstractions/src/Metrics/IMeterFactory.cs

@@ -4,7 +4,7 @@
 using System;
 using System.Diagnostics.Metrics;
 
-namespace Microsoft.Extensions.Metrics
+namespace Microsoft.Extensions.Diagnostics.Metrics
 {
     /// <summary>
     /// A factory for creating <see cref="Meter"/> instances.

src/libraries/Microsoft.Extensions.Diagnostics.Abstractions/src/Microsoft.Extensions.Diagnostics.Abstractions.csproj

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <EnableDefaultItems>true</EnableDefaultItems>

src/libraries/Microsoft.Extensions.Diagnostics/ref/Microsoft.Extensions.Diagnostics.cs

@@ -1,14 +1,14 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-namespace Microsoft.Extensions.Metrics
+namespace Microsoft.Extensions.Diagnostics.Metrics
 {
     public static class MetricsServiceExtensions
     {
         public static Microsoft.Extensions.DependencyInjection.IServiceCollection AddMetrics(this Microsoft.Extensions.DependencyInjection.IServiceCollection services) { return null!; }
     }
     public static class MeterFactoryExtensions
     {
-        public static System.Diagnostics.Metrics.Meter Create(this Microsoft.Extensions.Metrics.IMeterFactory meterFactory, string name, string? version = null, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, object?>>? tags = null,  object? scope = null) { return null!; }
+        public static System.Diagnostics.Metrics.Meter Create(this Microsoft.Extensions.Diagnostics.Metrics.IMeterFactory meterFactory, string name, string? version = null, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, object?>>? tags = null,  object? scope = null) { return null!; }
     }
 }

src/libraries/Microsoft.Extensions.Diagnostics/ref/Microsoft.Extensions.Diagnostics.csproj

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <CLSCompliant>false</CLSCompliant>

src/libraries/Microsoft.Extensions.Diagnostics/src/Metrics/DefaultMeterFactory.cs

@@ -0,0 +1,79 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.Metrics;
+
+namespace Microsoft.Extensions.Diagnostics.Metrics
+{
+    internal sealed class DefaultMeterFactory : IMeterFactory
+    {
+        private readonly Dictionary<string, List<Meter>> _cachedMeters = new();
+        private bool _disposed;
+
+        public DefaultMeterFactory() { }
+
+        public Meter Create(MeterOptions options)
+        {
+            if (options is null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
+            Debug.Assert(options.Name is not null);
+
+            lock (_cachedMeters)
+            {
+                if (_disposed)
+                {
+                    throw new ObjectDisposedException(nameof(DefaultMeterFactory));
+                }
+
+                if (_cachedMeters.TryGetValue(options.Name, out List<Meter>? meterList))
+                {
+                    foreach (Meter meter in meterList)
+                    {
+                        if (meter.Version == options.Version && DiagnosticsHelper.CompareTags(meter.Tags, options.Tags))
+                        {
+                            return meter;
+                        }
+                    }
+                }
+                else
+                {
+                    meterList = new List<Meter>();
+                    _cachedMeters.Add(options.Name, meterList);
+                }
+
+                Meter m = new Meter(options.Name, options.Version, options.Tags, scope: this);
+                meterList.Add(m);
+                return m;
+            }
+        }
+
+        public void Dispose()
+        {
+            lock (_cachedMeters)
+            {
+                if (_disposed)
+                {
+                    return;
+                }
+
+                _disposed = true;
+
+                foreach (List<Meter> meterList in _cachedMeters.Values)
+                {
+                    foreach (Meter meter in meterList)
+                    {
+                        meter.Dispose();
+                    }
+                }
+
+                _cachedMeters.Clear();
+            }
+        }
+    }
+}

src/libraries/Microsoft.Extensions.Diagnostics/src/MeterFactoryExtensions.cs → src/libraries/Microsoft.Extensions.Diagnostics/src/Metrics/MeterFactoryExtensions.cs

@@ -5,7 +5,7 @@
 using System.Collections.Generic;
 using System.Diagnostics.Metrics;
 
-namespace Microsoft.Extensions.Metrics
+namespace Microsoft.Extensions.Diagnostics.Metrics
 {
     /// <summary>
     /// Extension methods for <see cref="Meter" /> and <see cref="IMeterFactory" />.

src/libraries/Microsoft.Extensions.Diagnostics/src/MetricsServiceExtensions.cs → src/libraries/Microsoft.Extensions.Diagnostics/src/Metrics/MetricsServiceExtensions.cs

@@ -5,7 +5,7 @@
 using Microsoft.Extensions.DependencyInjection.Extensions;
 using System;
 
-namespace Microsoft.Extensions.Metrics
+namespace Microsoft.Extensions.Diagnostics.Metrics
 {
     /// <summary>
     /// Extension methods for setting up metrics services in an <see cref="IServiceCollection" />.
@@ -19,7 +19,7 @@ public static class MetricsServiceExtensions
         /// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
         public static IServiceCollection AddMetrics(this IServiceCollection services)
         {
-            if (services == null)
+            if (services is null)
             {
                 throw new ArgumentNullException(nameof(services));
             }

src/libraries/Microsoft.Extensions.Diagnostics/src/Microsoft.Extensions.Diagnostics.csproj

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <EnableDefaultItems>true</EnableDefaultItems>
@@ -11,6 +12,10 @@
     <PackageDescription>This package includes the default implementation of IMeterFactory and additional extension methods to easily register it with the Dependency Injection framework.</PackageDescription>
   </PropertyGroup>
 
+  <ItemGroup>
+    <Compile Include="$(CommonPath)System\Diagnostics\DiagnosticsHelper.cs" Link="Common\System\Diagnostics\DiagnosticsHelper.cs" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="$(LibrariesProjectRoot)System.Diagnostics.DiagnosticSource\src\System.Diagnostics.DiagnosticSource.csproj" />
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.Diagnostics.Abstractions\src\Microsoft.Extensions.Diagnostics.Abstractions.csproj" />

src/libraries/Microsoft.Extensions.Diagnostics/tests/MetricsTests.cs

@@ -4,7 +4,7 @@
 using Microsoft.DotNet.RemoteExecutor;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
-using Microsoft.Extensions.Metrics;
+using Microsoft.Extensions.Diagnostics.Metrics;
 using System.Collections.Generic;
 using System.Diagnostics.Metrics;
 using System.Linq;

src/libraries/System.Diagnostics.DiagnosticSource/src/System.Diagnostics.DiagnosticSource.csproj

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
@@ -70,6 +71,7 @@ System.Diagnostics.DiagnosticSource</PackageDescription>
     <Compile Include="System\Diagnostics\Metrics\UpDownCounter.cs" />
 
     <Compile Include="$(CommonPath)System\HexConverter.cs" Link="Common\System\HexConverter.cs" />
+    <Compile Include="$(CommonPath)System\Diagnostics\DiagnosticsHelper.cs" Link="Common\System\Diagnostics\DiagnosticsHelper.cs" />
 
     <None Include="DiagnosticSourceUsersGuide.md" />
     <None Include="ActivityUserGuide.md" />