Propagate `traceId` to native layer

[bitsandfoxes]

Problem Statement

The Unity SDK currently lacks proper trace ID propagation from the managed (C#) layer to the underlying native SDKs (Cocoa, Android, Native). This creates disconnected traces across different layers of the application, making it difficult to correlate events. There is no way to connect C# errors prior to a native event outside of breadcrumb messages.

Current Status by Platform

Native SDK

• ❌ No propagation context available on the scope
• ❌ Missing functionality to set trace context on the scope
• ❌ No continue trace API implementation
• Impact: Events from native crashes cannot be correlated with their originating managed traces

Android SDK

• ✅ Java errors are connected by a trace ID
• ❌ Native SDK trace context missing
• ❌ NDK Scope Observer not propagating trace ID
• Impact: Mixed trace propagation success; native crashes still disconnected

Cocoa SDK

• ❌ Missing continue trace API
• ❌ Trace propagation via bridge not functioning
• Impact: Crash events generate new trace IDs instead of continuing existing traces

.NET SDK

• ❌ Missing functionality for the Unity SDK to set and control the trace ID's lifecycle

Proposal

What we're working here right now is

1. Typically, we disable the performance feature on the native layer, keeping it to the "head" SDK.
2. This makes these SDKs fall back on the TwP, using the PropagationContext on the scope

We could add some internal SetTrace method that allows an SDK to set the trace on the native SDK's propagation context. This can then be observed and in turn, be propagated further down the layers.
We'd also need a way to disable any auto-trace-generation on those native SDKs.

Step by Step

Native SDK:

• Add trace context support to scope: feat: Added sentry_set_trace sentry-native#1137
• (Optional) Add continue trace API
• (Optional) Implement propagation context mechanism
• Let the Unity SDK propagate the trace ID to the native layer for Windows & Linux feat: Propagate traceId on platforms supported by sentry-native #2089

Android SDK:

• Implement trace context support in native layer feat: Added sentry_set_trace sentry-native#1137
• Prevent the Java SDK from generating trace IDs. This is achieved via a dedicated enableTraceIdGeneration to the AndroidOptions feat: Added enableTraceIdGeneration option sentry-java#4188
• Implement SetTrace feat: Allow Hybrid SDK to setTrace sentry-java#4137
• Extend NDK Scope Observer to handle trace ID propagation: feat: Allow Hybrid SDK to setTrace sentry-java#4137
• Let the Unity SDK propagate the trace ID on Android and further, to native feat: Propagate traceId to the Android SDK #1997

.NET SDK:

• Provide SetTrace functionality so the Unity SDK can take control over the trace ID generation feat: Allow IScopeObserver to observe trace sentry-dotnet#4026
• Extend ScopeObserver to handle trace ID propagation

Cocoa SDK:

• Prevent the Cocoa SDK from generating trace IDs. The SDK does this by creating a new propagationContext when creating a new scope.
• Implement SetTrace feat: Allow Hybrid SDKs to setTrace sentry-cocoa#5081
• Update bridge to use SetTrace. feat: Propagate traceId to the Cocoa SDK #2106
• Extend ScopeObserver to handle trace ID propagation. feat: Propagate traceId to the Cocoa SDK #2106

React Native SDK:

• Tracking that separately here Synchronize PropagationContext across layers sentry-react-native#3918

Expected Outcome

After implementation, all events (managed and native) should maintain consistent trace IDs throughout the entire crash reporting pipeline. This enables proper event correlation across all layers of Unity games.

[bitsandfoxes]

The first version containing the feature was released in 3.2.0.
The followup, extending this functionality to observe transactions and propagate their trace is done in getsentry/sentry-dotnet#4153 and waiting to be released and then bumped.

[bitsandfoxes]

Released as 3.2.2 🎉