[AzureMonitorExporter] Trace based logs sampler

[rajkumar-rangaraj]

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[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

Azure.Monitor.OpenTelemetry.AspNetCore
Azure.Monitor.OpenTelemetry.Exporter

[Copilot]

Pull Request Overview

This PR implements a trace-based logs sampler feature for Azure Monitor OpenTelemetry exporters. This feature filters out logs that are associated with unsampled traces while allowing logs without trace context to pass through, helping reduce log volume while maintaining correlation with sampled traces.

• Adds LogFilteringProcessor class that extends BatchLogRecordExportProcessor to filter logs based on trace sampling decisions
• Introduces EnableTraceBasedLogsSampler configuration option (default: true) in both AzureMonitorExporterOptions and AzureMonitorOptions
• Updates exporter registration logic to conditionally use LogFilteringProcessor based on the configuration flag

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
LogFilteringProcessor.cs	New processor that filters logs associated with unsampled traces by checking SpanId and TraceFlags
LogFilteringProcessorTests.cs	Comprehensive test suite validating filtering behavior for sampled, unsampled, and non-trace logs
AzureMonitorExporterOptions.cs	Adds EnableTraceBasedLogsSampler property to control the filtering feature
AzureMonitorOptions.cs	Adds EnableTraceBasedLogsSampler property and propagates it to exporter options
ExporterRegistrationHostedService.cs	Updates processor initialization to use LogFilteringProcessor when the feature is enabled
AzureMonitorExporterExtensions.cs	Updates extension methods to conditionally create LogFilteringProcessor based on configuration
AzureMonitorOptionsTests.cs	New test file verifying EnableTraceBasedLogsSampler property behavior and propagation
DefaultAzureMonitorOptionsTests.cs	Updates existing tests to verify the new property's default value
Azure.Monitor.OpenTelemetry.AspNetCore.csproj	Switches from PackageReference to ProjectReference for local development

[mattsains-msft]

other than the comment I made, I'm good with this PR. Check the CI though, seems to be failing. Tag me once the code documentation is improved and I can mark this as approved

[premun]

@rajkumar-rangaraj @xiang17 when we bumped from 1.5.0-beta.1 to the stable 1.5.0 version, our Azure Container Apps service stopped logging into Application Insights below Warning level.

I found out that this helps to bring the logs back but not sure this is intended behaviour:

.UseAzureMonitor(options =>
{
    // Disable trace-based log sampling to ensure all logs (including Information/Debug) are sent
    // This was changed in Azure.Monitor.OpenTelemetry.Exporter 1.5.0 which filters logs by default
    options.EnableTraceBasedLogsSampler = false;
})

We are not using sampling or any features really. Just the most basic appinsights integration and this was a bad breaking change for us somehow. But maybe we do something else wrong:

• https://github.com/dotnet/arcade-services/blob/df7ec0d8539be7de6d7c63bfadd59fe20b816875/src/ProductConstructionService/ProductConstructionService.ServiceDefaults/Extensions.cs#L90
• https://github.com/dotnet/arcade-services/blob/df7ec0d8539be7de6d7c63bfadd59fe20b816875/src/ProductConstructionService/ProductConstructionService.Api/appsettings.json#L2-L14

Do you know what went wrong here?