Micrometer context propagation for coroutines not working in endpoints that return `Flow`.

[sam-ov]

Given that automatic Micrometer context propagation for MDC is enabled in a WebFlux application,
When I call an endpoint that returns a Kotlin Flow,
Then I expect MDC values to be visible inside the flow-collector coroutine.

Actual: MDC values are only propagated correctly in suspend controller methods, not in Flow endpoints.

I'm using Spring Boot 4.0.3.

A complete app is in the linked repo (https://github.com/sam-ov/flow-context-propagation) but essentially I set up context propagation like this:

Hooks.enableAutomaticContextPropagation()
ContextRegistry.getInstance().registerThreadLocalAccessor(Slf4jThreadLocalAccessor())

When I call a suspend controller method, context propagation is working correctly. The coroutine context contains a PropagationContextElement, and the coroutine carries MDC context with it when it switches thread.

@GetMapping("/suspend")
suspend fun suspendEndpoint(): Something {
  // ✅ MDC propagation is working here
}

But when I call an endpoint that returns a Flow:

• The coroutine context does not contain PropagationContextElement
• The MDC context is either not present at all, or is lost when the coroutine suspends or switches thread

@GetMapping("/flow")
fun flowEndpoint(): Flow<Something> = flow {
  // ❌ MDC propagation is not working here
}

I would expect the coroutine that collects the Flow to have context propagation enabled in the same way as the coroutine that executes a suspend controller method, so I think this is a bug/omission in Spring's context propagation setup.

The minimal reproduction code is here: https://github.com/sam-ov/flow-context-propagation

The repository contains a passing test for the /suspend endpoint and a failing test for the /flow endpoint.

[sam-ov]

Maybe the solution is for Spring to add the PropagationContextElement to the coroutine context when calling asFlux() in ReactiveAdapterRegistry.CoroutinesRegistrar?

https://github.com/spring-projects/spring-framework/blob/main/spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java#L359