Fix NullReferenceException in WithMetrics when OTEL_SDK_DISABLED=true

[Copilot]

When OTEL_SDK_DISABLED=true, WithMetrics() crashes with NullReferenceException. The OpenTelemetryMetricsListener constructor receives a NoopMeterProvider from DI but assumes MeterProviderSdk, leading to null dereference when subscribing to events.

Changes

OpenTelemetryMetricsListener.cs

• Made meterProviderSdk field nullable (MeterProviderSdk?)
• Added null guards before event subscription in constructor, Dispose(), and InstrumentPublished()
• Suppressed CA1508 false positive (static analyzer can't see DI may inject noop provider)

Test coverage

• Added WithMetricsWhenSdkDisabledTest verifying no crash when SDK disabled
• Validates NoopMeterProvider returned and meters remain usable

Example

Environment.SetEnvironmentVariable("OTEL_SDK_DISABLED", "true");

var builder = Host.CreateApplicationBuilder();
builder.Services
    .AddOpenTelemetry()
    .WithMetrics(m => m.AddMeter("MyMeter"));  // Previously crashed, now works

var host = builder.Build();  // Success

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug] Null Reference Exception from within WithMetrics when OTEL_SDK_DISABLED is set to true</issue_title>
<issue_description>### Package

OpenTelemetry

Package Version

Package Name	Version
OpenTelemetry.Exporter.OpenTelemetryProtocol	1.15.0
OpenTelemetry.Extensions.Hosting	1.15.0
OpenTelemetry.Instrumentation.AspNetCore	1.15.0
OpenTelemetry.Instrumentation.Http	1.15.0
OpenTelemetry.Instrumentation.Runtime	1.15.0

Runtime Version

net10.0

Description

Application crashes on startup within the WithMetrics(x=>{}) Extension method when the Environment variable OTEL_SDK_DISABLED is set to true.

Steps to Reproduce

I have included the contents of a file-based app which I have been able to reproduce the issue

#!/bin/env dotnet
#:package Microsoft.Extensions.Hosting@10.0.2
#:package OpenTelemetry.Exporter.OpenTelemetryProtocol@1.15.0
#:package OpenTelemetry.Extensions.Hosting@1.15.0
#:package OpenTelemetry.Instrumentation.AspNetCore@1.15.0
#:package OpenTelemetry.Instrumentation.Http@1.15.0
#:package OpenTelemetry.Instrumentation.Runtime@1.15.0
#:property PathMap=$([System.IO.Path]::GetFullPath('$(MSBuildThisFileDirectory)'))=./

using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.DependencyInjection;
using OpenTelemetry.Logs;
using OpenTelemetry.Metrics;
using OpenTelemetry.Trace;

Environment.SetEnvironmentVariable("OTEL_SDK_DISABLED", "true");

var builder = Host.CreateApplicationBuilder(args);

builder.Services
    .AddOpenTelemetry()
    .WithMetrics(metricsProviderBuilder =>
    {
        metricsProviderBuilder
            .AddRuntimeInstrumentation()
            .AddAspNetCoreInstrumentation()
            .AddHttpClientInstrumentation()
            .AddOtlpExporter();
    })
    .WithTracing(tracingProviderBuilder =>
    {
        tracingProviderBuilder
            .AddSource("Test Application")
            .AddAspNetCoreInstrumentation()
            .AddHttpClientInstrumentation()
            .SetSampler<AlwaysOnSampler>()
            .AddOtlpExporter();
    });

builder.Build().Run();

Expected Result

Application should run with Open Telemetry SDK Disabled

Actual Result

Application exits after Object reference not set to an instance of an object

Additional Context

No response

Tip

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}</issue_description>

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

• Fixes [bug] Null Reference Exception from within WithMetrics when OTEL_SDK_DISABLED is set to true #6868

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (e7da73e) to head (a9539d6).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6869      +/-   ##
==========================================
- Coverage   87.29%   87.25%   -0.04%     
==========================================
  Files         263      263              
  Lines       12385    12387       +2     
==========================================
- Hits        10811    10808       -3     
- Misses       1574     1579       +5     

Flag	Coverage Δ
unittests-Project-Experimental	86.90% <100.00%> (-0.29%)	⬇️
unittests-Project-Stable	87.01% <100.00%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...s/IMetricsListener/OpenTelemetryMetricsListener.cs	100.00% <100.00%> (+3.44%)	⬆️

... and 7 files with indirect coverage changes

[Copilot]

Pull request overview

This PR fixes a NullReferenceException that occurred when OTEL_SDK_DISABLED=true is set. The issue was that OpenTelemetryMetricsListener assumed it would always receive a MeterProviderSdk instance but received a NoopMeterProvider when the SDK was disabled, causing null dereference on event subscriptions.

Changes:

• Made OpenTelemetryMetricsListener handle nullable MeterProviderSdk with appropriate null guards
• Added comprehensive test for SDK disabled scenario
• Enhanced test coverage for byte, short, int, and float numeric types across metrics tests

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/OpenTelemetry/Metrics/IMetricsListener/OpenTelemetryMetricsListener.cs	Made meterProviderSdk nullable and added null guards in constructor, Dispose(), and InstrumentPublished() to handle NoopMeterProvider
test/OpenTelemetry.Extensions.Hosting.Tests/OpenTelemetryMetricsBuilderExtensionsTests.cs	Added test for SDK disabled scenario and expanded test coverage to include byte, short, int, float, and double counter types
test/OpenTelemetry.Extensions.Hosting.Tests/OpenTelemetry.Extensions.Hosting.Tests.csproj	Added reference to EnvironmentVariableScope test helper
test/OpenTelemetry.Tests/Metrics/MetricApiTests.cs	Added multithreaded counter and histogram tests for byte, short, int, and float types
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.FuzzTests/Generators.cs	Updated fuzzing tests to include counters for all numeric types
src/OpenTelemetry.Extensions.Hosting/CHANGELOG.md	Documented the NullReferenceException fix

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