Is `MDCContext` (<https://github.com/Kotlin/kotlin...

# coroutines

[rocketraman]

Is MDCContext (https://github.com/Kotlin/kotlinx.coroutines/blob/master/integration/kotlinx-coroutines-slf4j/src/MDCContext.kt) implemented correctly? It wraps MDC which is a mutable thread local. The docs for ThreadContextElement seem to indicate that CopyableThreadContextElement should be used for this case.