Support Kotlin Coroutines

Currently Elastic Apm Java Agent doesn't support tracing context inside Kotlin Coroutines. And when you use Elastic Apm in Kotlin project, tracing doesn't work inside coroutines. So it would be quite useful to have Elastic Apm suppurt for Kotlin Coroutines.

I think the best solution is to implement Elastic Apm Java Agent Plugin for supporting Kotlin Coroutines.

Alternatively we can pass tracing context manually to kotlin coroutine context. But that doesn't allow other plugins to work correctly and adds a lot of boilerplate code.