Support Kotlin coroutines

[sjoerdtalsma]

Is your feature request related to a problem? Please describe.

Kotlin coroutines are stored in memory and have their own contexts and can be scheduled to resume on different threads.

ThreadLocal contexts won't work with coroutines out of the box.

Describe the solution you'd like

Support coroutines similar to the solution that was created for Slf4J MDC context: kotlinx-coroutines-slf4j

Describe alternatives you've considered

Additional context

• Documentation: https://github.com/Kotlin/kotlinx.coroutines/blob/master/docs/coroutine-context-and-dispatchers.md#thread-local-data

[sjoerdtalsma]

See Kotlin/kotlinx.coroutines#119

[codefromthecrypt]

this might help openzipkin/brave#820 (comment)

[sjoerdtalsma]

As I don't use Kotlin in production at this moment, I feel I would not be able to provide reasonable usecases for the tests to write.