feat: add OpenTelemetry trace correlation for LoggerFactoryLogger

[Aaronontheweb]

Summary

Implements GitHub issue #700: Add OpenTelemetry trace correlation support to LoggerFactoryLogger.

Problem: Activity.Current (used by OpenTelemetry) doesn't flow across actor mailbox boundaries because it uses AsyncLocal<T>. This means logs generated by actors lose their trace context correlation.

Solution: Leverage LogEvent.ActivityContext added in Akka.NET 1.5.59, which captures trace context at log event creation time before the mailbox crossing.

Changes

• AkkaLogState - Low-allocation struct using lazy enumeration (yield return) to propagate trace context in MEL state dictionaries. Stores ActivityTraceId/ActivitySpanId as structs directly to avoid ToString() allocations.

• AkkaTraceContextProcessor - BaseProcessor<LogRecord> that extracts Akka.TraceId, Akka.SpanId, Akka.TraceFlags from log attributes and sets them on LogRecord for OpenTelemetry exporters.

• AddAkkaTraceCorrelation() - Extension method on OpenTelemetryLoggerOptions for easy configuration.

• LoggerFactoryLogger - Updated to include ActivityContext in log state when available.

• Dependencies - Bumped Akka.NET to 1.5.59, Microsoft.Extensions to 8.0.0

• Aspire Demo - Added demo project (src/Examples/Akka.Hosting.OpenTelemetry.Demo/) with Seq for visual validation

Usage

builder.Logging.AddOpenTelemetry(options =>
{
    options.SetResourceBuilder(ResourceBuilder.CreateDefault()
        .AddService("my-service"));

    // Register processor FIRST (before exporters)
    options.AddAkkaTraceCorrelation();

    options.AddOtlpExporter();
});

services.AddAkka("MySystem", configBuilder =>
{
    configBuilder.ConfigureLoggers(setup =>
    {
        setup.ClearLoggers();
        setup.AddLoggerFactory();
    });
});

Test plan

• Unit tests for AkkaLogState (8 tests)
• Unit tests for AkkaTraceContextProcessor (10 tests)
• All existing tests pass (297 tests)
• Manual validation with Aspire demo

Closes #700

[Aaronontheweb]

Validated this approach works for Phobos log/trace correlation.

We created a prototype test using the AkkaLogState and AkkaTraceContextProcessor implementations from this PR to verify that trace context correctly propagates through the MEL state dictionary. The test confirms that TraceId, SpanId, and TraceFlags are all properly captured and accessible.

This will allow us to deprecate Phobos's AppendLogsToTrace feature (which was a workaround for this exact problem) and point users to LoggerFactoryLogger + AddAkkaTraceCorrelation() as the proper solution.

[Aaronontheweb]

Some screenshots from the Aspire demo:

Traces

Logs Correlated to Trace

These were all captured via the ILoggingAdapter

[Aaronontheweb]

Reviewed some of the changes here, have some fixes to make

[Aaronontheweb]

While all of these logging changes are still new, want to be careful about not err-ing out if there are formatting issues / config conflicts

[Aaronontheweb]

this is true unfortunately, per the path the OTEL authors recommended to us open-telemetry/opentelemetry-dotnet#6085

[Aaronontheweb]

Going to update this to use UnsafeAccessor instead

[Aaronontheweb]

lol can't - due to .NET Framework

[Aaronontheweb]

Detailed my changes - good to go for a beta release IMHO

[Aaronontheweb]

The OTEL packages require these

[Aaronontheweb]

Added documentation

[Arkatufus]

LGTM